What is AIDS?

AIDS (acquired immune deficiency syndrome) is caused by the human immunodeficiency virus (HIV), which kills or impairs cells of the immune system and progressively destroys the body's ability to fight infections and certain cancers. HIV is most commonly spread by sexual contact with an infected partner.

The term AIDS applies to the most advanced stages of an HIV infection. Official criteria for the definition of AIDS are developed by the Centers for Disease Control and Prevention (CDC), which is responsible for tracking the spread of AIDS in the United States. The 2008 CDC definition of AIDS includes all HIV-infected people who have fewer than 200 CD4+ T cells (healthy adults usually have CD4+ T-cell counts of 800 or more.) In addition, the definition includes HIV-infected people who have been diagnosed with one or more of 26 clinical conditions (opportunistic infections) that affect people with advanced HIV disease.

According to the CDC, in 2010 more than 1.1 million persons aged 13 years and older were living with an HIV infection in the United States including 207,600 (18%) who were unaware of their infection. An estimated 2.7 million new HIV infections occurred worldwide during 2007, which equates to about five new infections every minute. This indicates that the AIDS epidemic still rages out of control. According to the World Health Organization (WHO), at the end of 2008, there were 33.4 million people living with AIDS globally.

How is HIV/AIDS Transmitted?

Sexual contact

HIV is spread most commonly by sexual contact with an infected partner. The virus enters the body through the lining of the vagina, vulva, penis, rectum, or mouth during sexual activity.

Blood contamination

HIV may also be spread through contact with infected blood. However, due to the screening of blood for evidence of HIV infection, the risk of acquiring HIV from blood transfusions is extremely low.


HIV is frequently spread by sharing needles, syringes, or drug use equipment with someone who is infected with the virus. Transmission from patient to healthcare worker, or vice-versa through accidental sticks with contaminated needles or other medical instruments, is rare.


HIV also can be spread to babies born to, or breastfed by, mothers infected with the virus.

HIV/AIDS cannot be spread through:

Putting a number on it: The risk from an exposure to HIV

This information was provided by CATIE (the Canadian AIDS Treatment Information Exchange). For more information, contact CATIE at 1.800.263.1638.
By James Wilton

Service providers working in HIV prevention are often asked by their patients and clients about the risk of HIV transmission from an exposure to HIV through sex. What do the latest studies tell us about this risk? And how should we interpret and communicate the results?

Challenges in calculating a number

It isn’t easy for researchers to calculate the risk of transmission from an exposure to HIV through sex. To do this effectively, a group of HIV-negative individuals need to be followed over time and their exposures to HIV—both the number of times they are exposed and the types of exposure—need to be tracked.

As you can imagine, accurately tracking the number of times a person is exposed to HIV is very difficult. Researchers ask HIV-negative individuals enrolled in these studies to report how many times they have had sex in a given period of time, what type of sex they had, how often they used condoms and the HIV status of their partner(s). Because a person may have trouble remembering their sexual behaviour or may not want to tell the whole truth, this reporting is often inaccurate.

Furthermore, a person does not always know the HIV status of their partner(s). For this reason, researchers usually enroll HIV-negative individuals who are in stable relationships with an HIV-positive partner (also known as serodiscordant couples). Researchers can then conclude that any unprotected sex reported by a study participant counts as an exposure to HIV.

Several studies have aimed to estimate the average risk of HIV transmission from a specific type of unprotected sex (for example, vaginal/anal/oral; insertive/receptive). Due to the difficulties of calculating this risk, these studies have produced a wide range of numbers. To come up with a more accurate estimate for each type of unprotected sex, some researchers have combined the results of individual studies into what is known as a meta-analysis.

All exposures are not equal

The results of several meta-analyses suggest that some types of sex carry on average a higher risk of HIV transmission than others. Below are estimates from meta-analyses that have combined the results of studies conducted in high-income countries. For types of sex where meta-analysis estimates do not exist, numbers from individual studies are provided.

Anal sex

A meta-analysis exploring the risk of HIV transmission through unprotected anal sex was published in 2010.1 The analysis, based on the results of four studies, estimated the risk through receptive anal sex (receiving the penis into the anus, also known as bottoming) to be 1.4%. (This means that an average of one transmission occurred for every 71 exposures.) This risk was similar regardless of whether the receptive partner was a man or woman.

No meta-analysis estimates currently exist for insertive anal sex (inserting the penis into the anus, also known as topping) but two individual studies were conducted to calculate this risk. The first, published in 1999, calculated the risk to be 0.06% (equivalent to one transmission per 1,667 exposures).2 However, due to the design of the study, this number likely underestimated the risk of HIV transmission. The second study, published in 2010, was better designed and estimated the risk to be 0.11% (or 1 transmission per 909 exposures) for circumcised men and 0.62% (1 transmission per 161 exposures) for uncircumcised men.3

Vaginal sex

A meta-analysis of 10 studies exploring the risk of transmission through vaginal sex was published in 2009.4 It estimated the risk of HIV transmission through receptive vaginal sex (receiving the penis in the vagina) to be 0.08% (equivalent to 1 transmission per 1,250 exposures).

A meta-analysis of three studies exploring the risk from insertive vaginal sex (inserting the penis into the vagina) was estimated to be 0.04% (equivalent to 1 transmission per 2,500 exposures).4

Oral sex

No meta-analysis estimates exist for oral sex (vaginal or penile) because too few good-quality studies have been completed. This is because it is difficult to find people whose only risk of HIV transmission is unprotected oral sex. A review of the studies that are available was published in 2008 and concluded that vaginal and penile oral sex pose a “low but non-zero transmission probability.”5

In the three studies aimed at calculating the risk of HIV transmission from one act of oral sex, no transmissions were observed among three different populations—lesbian serodiscordant couples, heterosexual serodiscordant couples and single gay men—who reported unprotected oral sex as their only risk for HIV transmission. However, these studies enrolled only a small number of people and followed them for only a short period of time, which may explain the lack of HIV transmissions and makes it impossible to conclude that the risk from oral sex is zero.

Risk of HIV transmission from different types of unprotected sex

Number of individual studies

Range of estimates

Meta-analysis estimate

Receptive anal




Insertive anal




Receptive vaginal




Insertive vaginal




Interpreting the numbers—what additional information needs to be provided?

Some clients may see these numbers and think their risk of HIV transmission is low. Therefore, caution is needed when interpreting them. If these numbers are provided to clients, they should be accompanied by information that helps shed light on why the risk may be higher than it seems.

Transmission can occur after one exposure.

It is important to emphasize that a person could become infected from having unprotected sex once or a person could have unprotected sex many times and not become infected, regardless of how low or high the risk per exposure is.

A risk of 1% would mean that an average of one infection would occur if 100 HIV-negative people were exposed to HIV through a certain type of sex. It does not mean that a person needs to be exposed 100 times for HIV infection to occur.

These are estimates of average risk in the absence of biological factors that increase risk.

The numbers in the table above are rough estimates. They are averages and do not represent the risk from all exposures to HIV through a certain type of sex.

We know that no two exposures to HIV are exactly the same. Research shows that, in addition to the type of sex that led to the exposure, several factors can increase or decrease the risk that an exposure to HIV leads to infection. These include the presence of sexually transmitted infections (STIs), a high viral load, a man being uncircumcised, a woman menstruating, other bleeding and activities that can cause tearing and inflammation, such as rough sex, longer sex, douching, enemas before anal sex, and tooth brushing, flossing or dental work before oral sex. Each exposure to HIV carries a unique risk of transmission that depends on the type of sex and a combination of biological factors.

The risk of HIV transmission may be much higher than these averages if biological risk factors are present. For example, research shows that STIs and some vaginal conditions, such as bacterial vaginosis, can increase the risk of HIV transmission by up to 8 times.6,7,8 As a result, the risk of an HIV-negative woman becoming infected through unprotected receptive vaginal sex could be closer to 1% (1 transmission per 100 exposures) if she has a vaginal STI.

We also know that for every 10-fold increase in viral load, the risk of HIV transmission increases by 2 to 3 times.9,10 Research suggests the extremely high viral load during acute HIV infection (the first few weeks after becoming infected with HIV) can increase the risk of HIV transmission by up to 26-fold.11,12 Therefore, unprotected sex with an HIV-positive person who has acute HIV infection could carry a transmission risk of up to 2% (the equivalent of 1 transmission per 50 exposures) for receptive vaginal sex and over 20% (equivalent to 1 transmission per 5 exposures) for receptive anal sex.

The more exposures, the greater the risk.

Although the risk of HIV transmission from a single exposure may seem low to some people, this risk increases over multiple exposures. In other words, a person who is exposed to HIV more often has a greater overall risk of HIV transmission than someone who is exposed less often.

If a woman has unprotected vaginal sex 100 times with a man who is HIV-positive, the cumulative risk is approximately 10% and may be higher if biological risk factors are present.

Differences in risk

Information on how risky certain types of unprotected sex are compared to others may help people make more informed decisions about the type of sex they are having.

Based on the meta-analysis estimates, we can draw several conclusions:


Although it’s impossible to provide a client with their exact risk of HIV transmission from an exposure, some studies have managed to estimate an average risk for different types of sex. It's important to provide clients with additional information to help them interpret the findings. Here are some key messages:

  1. These numbers
    1. are challenging to calculate and should therefore be considered rough estimates
    2. do not represent the risk of transmission from all exposures to HIV
    3. represent the average risk of transmission in the absence of biological factors that can increase risk (such as STIs and a high viral load)
    4. are most relevant to people in stable monogamous serodiscordant relationships
  2. These numbers may seem low but
    1. HIV transmission can occur after a single exposure
    2. the risk may be much higher if certain biological risk factors, such as STIs or a high viral load, are present
    3. as more exposures to HIV occur, the overall risk of transmission increases
    4. most HIV transmissions in Canada occur through unprotected anal and vaginal sex
  3. There are several ways of reducing the risk of HIV transmission from an exposure, such as post-exposure prophylaxis (PEP), using antiretroviral treatment to reduce viral load, circumcision, treatment for STIs and vaginal conditions, or engaging in lower-risk activities.
  4. There is no way to reduce the risk of HIV transmission to zero after an exposure occurs. Taking measures to avoid an exposure in the first place (for example, through the correct use of condoms or other barrier methods, or by ensuring a partner has the same HIV status) can help reduce the overall risk of HIV transmission.


1. Baggaley RF, White RG, Boily M-C. HIV transmission risk through anal intercourse: systematic review, meta-analysis and implications for HIV prevention. International Journal of Epidemiology. 2010 Aug;39(4):1048–63.

2. Vittinghoff E, Douglas J, Judson F et al. Per-contact risk of human immunodeficiency virus transmission between male sexual partners. American Journal of Epidemiology. 1999 Aug 1;150(3):306–11.

3. Jin F, Jansson J, Law M et al. Per-contact probability of HIV transmission in homosexual men in Sydney in the era of HAART. AIDS. 2010 Mar 27;24(6):907–13.

a. b. Boily M-C, Baggaley RF, Wang L et al. Heterosexual risk of HIV-1 infection per sexual act: systematic review and meta-analysis of observational studies. Lancet Infectious Diseases. 2009 Feb;9(2):118–29.

5. Baggaley RF, White RG, Boily M-C. Systematic review of orogenital HIV-1 transmission probabilities. International Journal of Epidemiology. 2008 Dec;37(6):1255–65.

6. Ward H, Rönn M. Contribution of sexually transmitted infections to the sexual transmission of HIV. Current Opinion in HIV and AIDS. 2010 Jul;5(4):305–10.

7. Atashili J, Poole C, Ndumbe PM et al. Bacterial vaginosis and HIV acquisition: a meta-analysis of published studies. AIDS. 2008 Jul 31;22(12):1493–501.

8. Cohen CR, Lingappa JR, Baeten JM et al. Bacterial vaginosis associated with increased risk of female-to-male HIV-1 transmission: a prospective cohort analysis among African couples. PLoS Medicine. 2012 Jun;9(6):e1001251.

9. Wawer MJ, Gray RH, Sewankambo NK et al. Rates of HIV-1 transmission per coital act, by stage of HIV-1 infection, in Rakai, Uganda. Journal of Infectious Diseases. 2005 May 1;191(9):1403–9.

10. Baeten JM, Kahle E, Lingappa JR et al. Genital HIV-1 RNA predicts risk of heterosexual HIV-1 transmission. Science Translational Medicine. 2011 Apr 6;3(77):77ra29.

11. Wawer MJ, Gray RH, Sewankambo NK et al. Rates of HIV-1 transmission per coital act, by stage of HIV-1 infection, in Rakai, Uganda. Journal of Infectious Diseases. 2005 May 1;191(9):1403–9.

12. Hollingsworth TD, Anderson RM, Fraser C. HIV-1 transmission, by stage of infection. Journal of Infectious Diseases. 2008 Sep 1;198(5):687–93.

About the author(s)

James Wilton is the Project Coordinator of the Biomedical Science of HIV Prevention Project at CATIE. James has an undergraduate degree in Microbiology and Immunology from the University of British Columbia.

HIV Transmission

Last Modified: March 25, 2010
Last Reviewed: March 25, 2010
Content Source:
Divisions of HIV/AIDS Prevention
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention

HIV/AIDS Symptoms

Some people may develop a flu-like illness within a month or two after exposure to the HIV virus, although, many people do not develop any symptoms at all when they first become infected. In addition, the symptoms that do appear, which usually disappear within a week to a month, are often mistaken for those of another viral infection. These may include:

Persistent or severe symptoms may not surface for 10 years or more, after HIV first enters the body in adults, or within two years in children born with an HIV infection. This "asymptomatic" period of the infection is highly variable from person to person. But, during the asymptomatic period, HIV is actively infecting and killing cells of the immune system. Its most obvious effect is a decline in the blood levels of CD4+ T cells (also called T4 cells)--the immune system's key infection fighters. The virus initially disables or destroys these cells without causing symptoms.

As the immune system deteriorates, complications begin to surface. The following are the most common complications, or symptoms, of AIDS. However, each individual may experience symptoms differently. Symptoms may include:

Some people develop frequent and severe herpes infections that cause mouth, genital, or anal sores, or a painful nerve disease known as shingles. Children may have delayed development or failure to thrive.

During the course of the HIV infection, most people experience a gradual decline in the number of CD4+ T cells, although some individuals may have abrupt and dramatic drops in their counts.

The symptoms of an HIV infection may resemble other medical conditions. Always consult your doctor for a diagnosis.

HIV/AIDS Diagnosis

Early HIV infection often causes no symptoms, and must be detected by testing a person's blood for the presence of antibodies--disease-fighting proteins--against HIV. These HIV antibodies generally do not reach levels high enough to detect by standard blood tests until one to three months following infection, and may take as long as six months. People exposed to HIV should be tested for HIV infection as soon as they are likely to develop antibodies to the virus.

When a person is highly likely to be infected with HIV and yet antibody tests are negative, a test for the presence of HIV itself in the blood is used. Repeat antibody testing at a later date, when antibodies to HIV are more likely to have developed, is often recommended.

HIV Drug Resistance Testing

Drug resistance testing is used to determine whether a patient with HIV has a mutated form of the virus that does not respond to antiretroviral therapy (ART).

If an HIV-infected patient becomes resistant to a drug and continues to take the same medication, HIV is able to multiply faster because the drug cannot stop it from replicating. When the new, mutated form is favored, it is called selective pressure. If the resistant virus makes enough copies of itself, it may eventually become the dominant type of HIV in the body. Once this happens, the medication is ineffective, and the patient will be resistant to the specific medication.

The FDA has approved the drug resistance test TrueGene™. The sensitivity, specificity, and reproducibility for many other tests have not been well established. However, several tests are available, and many health insurance plans cover them. There are two main types of drug resistance tests: genotypic and phenotypic resistance tests.

Genotypic resistance testing examines the genetic structure (genotype) of a patient's HIV. A blood sample is taken from the patient, and the HIV is analyzed for the presence of specific genetic mutations that are known to cause resistance to specific drugs. For instance, researchers have determined that lamivudine (Epivir®) and emtricitabine (Emtriva®) are not effective against forms of HIV that contain the mutation "M184V" in its reverse transcriptase gene. If a patient tests positive for this mutation, it is highly likely that he or she is resistant to both drugs, and different drugs should be prescribed.

Phenotypic testing directly measures the sensitivity (phenotype) of a patient's HIV in response to specific antiretrovirals. Many experts believe that these tests are more accurate and comprehensive than genotypic tests. These tests can help a physician determine the amount or concentration of a drug that is needed to stop a specific strain of HIV from replicating in a patient.

HIV/AIDS Treatment

Today, there are also medical treatments that can slow down the rate at which HIV weakens the immune system, but currently there is no cure for the disease. However, there are other treatments that can prevent or cure the conditions associated with AIDS except few rare case reports under research. Consult your doctor for more information regarding various drug therapies for the treatment of HIV/AIDS.

Antiretroviral Drugs

Although current antiretroviral drugs cannot cure HIV/AIDS, they may suppress the virus, even to undetectable levels. The guidelines for antiretroviral treatment are the same for adolescents and adults.

Patients who were taking antiretroviral medication before becoming pregnant should talk to their healthcare providers to determine the safest and most effective treatment options.

Organ transplants

As mentioned above, HAART enables HIV/AIDS patients to live longer lives. Today, most patients with HIV/AIDS are dying from end-stage organ disease and organ failure rather than AIDS-associated opportunistic infections. Since HAART prolongs the lives of HIV patients, it is possible for chronic conditions to progress to organ failure. For instance, HIV patients may experience end-stage liver disease as a complication of chronic hepatitis C virus. Glomeruli diseases are also common among HIV patients, and they may lead to kidney failure. In advanced stages of liver or kidney damage, organ transplants may be the patient's only chance of survival.

Until recently, people who had HIV were not considered good candidates for organ transplantations. Many patients were denied transplants under the assumption that they had shorter life expectancies and less favorable survival rates than other patients in need of transplants. However, now that patients are living longer lives, many groups are re-considering whether HIV patients should be transplant candidates.

Although the United Network for Organ Sharing (UNOS) does not consider HIV infection a contraindication for organ transplantation, individual transplant centers are in charge of deciding whether or not to perform surgery in an HIV-positive patient. Some centers will not provide organ transplants to HIV-positive patients, even if they are good candidates based on their physical and mental health.

Some health insurance companies are reluctant to cover transplantation in HIV-positive candidates because they consider it to be an experimental procedure. Currently, only a few medical centers worldwide perform organ transplants in HIV-positive patients. However, health insurance companies and doctors consider organ transplantations in HIV-negative patients to be a well-established, reimbursable procedure.

Recent legislation in California and a ruling in Arizona may help increase HIV patients' access to transplant surgery. In October 2005, an administrative law judge declared that Medicaid had to pay for a liver transplant for an Arizona woman who was HIV-positive. In the same month, California Governor Arnold Schwarzenegger signed a law that prohibits health insurance companies from denying coverage for organ transplants in HIV patients solely on the basis of their HIV-status. The law is the first of its kind to target such denials.

The limited number of transplants that have been performed in HIV patients have produced encouraging results. However, organ transplants for people with HIV/AIDS have not gained widespread medical support, and there are still concerns regarding the long-term prognosis for HIV-positive transplant recipients.

Preventing Opportunistic Infections in HIV/AIDS

HIV attacks the cells of your body's immune system. You need a strong immune system to fight off germs like bacteria and viruses, so having HIV may give those germs a better opportunity to make you sick. When germs take advantage of your weakened defense system, they are called opportunistic infections (OIs).

Opportunistic infections that other people might fight off easily could make you really sick if you have HIV. Getting one or more of these OIs could mean that your HIV has advanced to AIDS. In fact, opportunistic infections are the most common cause of death for people with HIV/AIDS. The good news is that you have plenty of ways to prevent them.

Opportunistic infections you need to know about

The CDC has made a list of more than 20 serious diseases that can become OIs if you have HIV/AIDS. You might have one of these diseases and be healthy enough to fight it off normally, but if it is hard to get rid of and lasts too long, it is considered an OI.

If you have one or more of the diseases on the CDC's list, you could be considered to have AIDS – that's why the CDC calls them AIDS-defining conditions. Here are the most common OIs:

Other OIs include lymphoma, encephalopathy (AIDS dementia), and wasting syndrome, often marked by weight loss, ongoing fever, diarrhea, and malnutrition.

HIV/AIDS Resources

Stanford Medicine Resources:

Footer Links: