The most common route of spread of HIV infection is through unprotected sex between two people, one of who has HIV infection. Unprotected sex means having sex without a condom. Unprotected vaginal sex, where the penis is inserted into the vagina is more common source of infection in India as compared to unprotected anal sex where the penis is inserted into the anus. Penis is the external reproductive organ of a man. HIV is present in the sperms as well as the seminal fluids. Sperms are the mature male "seeds" or the germ cells. When a sperm "unites" with the female egg (ovum), fertilisation occurs. Seminal fluid is thick, whitish secretions of the male reproductive organs that is discharged from the body during ejaculation.

Even one episode of unprotected sex with an infected partner can transmit HIV.

The risk of HIV being transmitted from an individual sexual act depends upon several factors. Increased number of unprotected sex with an infected partner increases the risk of infection.

Women are at a 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 the delicate outer 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, the opening through which semen and urine is passed out of the body. Larger surface area provides greater opportunity for the virus to enter the body.
  3.      A large number of women who have infections of their reproductive tract may not have any symptoms at all. In the absence of any symptoms, women will not know they have the infections and will therefore not take appropriate treatment. These infections allow greater opportunity for the HIV to enter the body and cause infection.

Sexually transmitted diseases: Any ulcer, injury or damage to the outer wall of the penis or vagina can increase the risk of getting HIV infection. Several sexually transmitted diseases cause ulcers or sores on the penis or walls of the vagina. The HIV can therefore easily enter the body through the damage to the genital skin caused by these ulcers and sores. Also, the whitish secretions due to sexually transmitted diseases contain large number of white blood cells. Large number of white blood cells mean large number of the virus in this pus. Since increased number of viruses in the body fluids increases the risk of transmitting HIV infection, a person with sexually transmitted diseases has a higher risk of transmitting HIV infection to the sexual partner.

Anal sex: This is the term used when the penis is inserted into the anus of the sexual partner. Although anal sex is more common among homosexuals, a large number of heterosexual partners also practice it. Anal sex has a higher risk of transmitting HIV infection as compared to vaginal sex. This is mainly because the inner lining of the anus is delicate and therefore gets damaged very easily during anal sex. This damage helps the HIV to enter the body and cause infection.

Homosexual sex: This is the term used when people prefer to have sex with partners of their own sex. Until recently, cultural taboos had prevented people from talking about homosexuality openly. Increased awareness about HIV/AIDS has resulted in more and more people willing to talk about homosexuality. Several studies have indicated that the number of homosexual males is higher in India that what was thought to be until a few years ago. A large number of homosexual males in India are married and have children. They therefore consider themselves as heterosexual people who have occasional sex with male partners. Majority of the sexual partners of eunuchs (hijras) are men. Although there are several people who are homosexual in India, HIV infection has not been widely reported among them. The main route of transmission of HIV continues to be heterosexual sex. This does not mean that there is low risk of getting HIV infection through homosexual sex in India. It means that at present there is higher risk in heterosexual sex mainly because people practising heterosexual sex is far more than those practicing homosexual sex.

Female homosexual sex (called lesbianism) is considered to have a very low risk of spreading the HIV infection. This is mainly because homosexual sex among women is non-penetrative.

Menstruation: The menstrual blood of a woman with HIV infection will contain the virus. Thus, sexual intercourse during menstruation with an infected woman increases the risk of the male partner getting HIV infection.