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What determines the chances of infection?

May 7, 1998

What things can detemine whether a person gets infected or not?

Response from Mr. Sowadsky

Hi. Thank you for your question. When we are talking about the likelihood of HIV transmission, there are numerous variables that can determine the chances of transmission. Let me review with you some of these variables.

1) The amount of the virus in the body (also known as the viral load) can affect the chances of infection. The higher the viral load, the greater the chances of transmission. The viral load is can vary from one person to another, from one body fluid to another, and even from one stage of the disease to another. Body fluids like saliva, tears, and urine, normally have extremely low concentrations of the virus at all times throughout the disease. However, body fluids like blood, semen, and vaginal secretions can have very high concentrations of the virus (especially as the disease progresses), but the viral loads in these body fluids can sometimes be reduced if a person responds well to treatment. Even with a very low viral load, transmission can still occur, although the likelihood of transmission is reduced (but not eliminated). Other than the low risk body fluids like saliva, tears, and urine, the viral load in body fluids like blood, semen, and vaginal secretions, can be highly variable throughout the disease, depending on the individual, the stage of the disease, and how well the person is responding to therapy.

2) The likelihood of transmission also depends on the route of transmission. Some activities (like unprotected intercourse and sharing needles) are more likely to lead to transmission, while other activities (like receiving oral sex and kissing) are very unlikely to lead to transmission.

3) The greater the amount of pre-cum, semen, vaginal secretions, or blood that you are exposed to, the greater the risk of infection. For example, it has been reported that HIV can be transmitted through pre-cum alone. But there is a much greater risk of becoming infected through semen, since you are normally exposed to a much greater quantity of semen, as compared to pre-cum. Both pre-cum and semen have the virus in them and both have been reported to transmit HIV. However semen is a much greater risk, since you are normally being exposed to a lot more semen than pre-cum.

4) The presence of other STDs can also determine the level of risk. We now know that some STDs make transmission of the HIV virus more likely to occur. See the article, The HIV-STD Connection, for more information about the link between STDs and HIV transmission.

5) The genetic make-up of a person can also affect their chances of transmission. We now know that there are multiple receptors that help HIV attach to host cells. These receptors include CD4, CCR-5 (also known as CKR-5), fusin, and others. If a person has certain rare genetic mutations in these receptors, the person may be less likely to become infected (although infection is still possible). There is no evidence that these rare genetic mutations will totally prevent infection. For more information on this topic, see the posting, Genetically not predisposed to HIV infection.

6) Other factors that can determine the chances of infection include low long a person engages in a risky activity, how many times they engage in that activity, and how many different partners they have. The more exposures you have, the greater the chance of infection. You can become infected after just one exposure to the virus. Likewise, you can be exposed multiple times and still not be infected with the virus. But the more times you are exposed, the greater the chance of infection.

In summary, because of all of these variables, the chances of infection can vary from situation to situation. When we talk about the chances of infection, we can realistically only talk in relative terms. We cannot give anyone a percentage of their risk, nor can we statistically quantitate their risks. In other words, the risks of infection are variable, depending on the specific circumstances.

If you have any further questions, please feel free to call the Centers for Disease Control at 1.800.232.4636 (Nationwide).

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