HIV infection spreads through 3 main routes:

  1. Direct - sexual intercourse or deep kissing. Oral or anal sex also leads to transmission of virus from one partner to the other
  2. Indirect – body fluids of the infected person like blood, vomit, excreta etc and infected syringes. HIV is present in all body fluids of an infected person. It is however more in number in blood, semen and vaginal fluids.
  3. Vertical – mother to child transmission during pregnancy or delivery.

More than 75% people are estimated to get it through sexual transmission.

The probability of getting the infection, with one encounter, is not the same by all routes.


Four mains conditions must be fulfilled if HIV is to be transmitted through any one of the three routes mentioned above. These includes:

  1. HIV must be present in the body fluids, especially the semen, vaginal fluids, blood or blood products.
  2. HIV must live during the period it is out of the body. It can live for a long time in blood stored at cold temperatures for transfusion but lives for a short time in all other situations as the body fluids dry very easily. HIV cannot survive in dried body fluids.
  3. There must be convenient portal for entry. The normal skin forms a very effective barrier and HIV will find it difficult to enter through intact skin. The virus can easily enter the body from wherever there is damage to the skin or the skin is more delicate (such as vagina or anus).
  4. The number of viruses in the body fluids must be adequate to infect others after it is transferred to contact with body fluids. If the number of viruses that enter another person is less, then the infection may not occur.


The most common (75%) route of the spread of HIV infection is through unprotected sex (without condom) between two people, one of whom already has HIV infection. Unprotected vaginal sex, is a more common source of infection in India as compared to unprotected anal sex. HIV is present in the sperms as well as the seminal fluids.

Even one episode of unprotected sex with an infected partner can transmit HIV. Multiple such episodes increase the risk of infection.

Women are at greater risk of developing HIV infection through unprotected sex. This means that the risk of transmission of HIV from man to woman is higher than that from woman to man. There are several reasons why women are at greater risk.

  1. The semen from the infected male sexual partner remains in the woman’s vagina for a longer time. Longer contact between infected semen and delicate wall of the vagina increases the risk of HIV infection.
  2. The surface area of the vagina is very large compared to the urethra in men. Larger surface area provides greater opportunity for the virus to enter the body.
  3. A large number of women who have sexually transmitted infections of their reproductive tract may not have any symptoms at all. In the absence of any symptoms, they will not know they have the infections and will therefore not take appropriate treatment. These STDs allow greater opportunity for the HIV to enter the body and cause infection.
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