The biggest risk factor for contracting human immunodeficiency virus (HIV) is engaging in unsafe sexual activity. This can be defined as having vaginal, oral or anal intercourse with someone whose HIV status is unknown. Additionally, intravenous drug users are at a greater risk of HIV infection than those who do not use such drugs.    

The methods of HIV transmission define the groups that are at increased risk of contracting HIV. These transmission methods include:

• Sexual contact. An infected person can transmit the virus to a sexual partner through semen or vaginal secretions that enter the partner’s body. Either gender can be infected by either a man or a woman. Vaginal, anal and oral sex are all potential avenues of transmission. Shared sexual devices can also transmit the virus when the device is not disinfected or covered with a condom.
  

  

  The virus usually enters the body of a sexual partner through tiny tears in the vagina or rectum. The likelihood of transmission greatly increases in people who already have another sexually transmitted disease. Women should also be aware that the spermicide nonoxynol-9 may irritate the lining of the vagina, causing tears that can increase the risk of transmission.
  
  The increased risk through sexual contact occurs in people with multiple sex partners, high risk sex partners and people who do not practice safe sex. In the United States, the majority of sexual transmission of HIV has occurred among men who have sex with men. In other parts of the world, such as Africa, most HIV transmission occurs during heterosexual sex.
  
  
• Tainted needles. HIV is easily transmitted through needles and syringes that have come into contact with infected blood and are then used by someone who does not yet have the virus. Drug addicts who share intravenous drug paraphernalia are especially at risk for this method of transmission. Healthcare workers and others are also sometimes infected when they are accidentally stuck with a needle that has been in contact with an infected person.
  
  
• Mother-to-child transmission. Women can transmit HIV to their children through pregnancy, delivery or breastfeeding. However, this risk declines significantly when mothers receive treatment for their HIV infection. In the United States, pregnant women are routinely screened for HIV infection. Those who test positive are prescribed anti-retroviral drugs. This screening has greatly reduced the incidence of pediatric AIDS cases in the United States.
  
  
• Infected blood. HIV can be transmitted through blood and blood products used in blood transfusions. Blood products include whole blood, packed red blood cells, fresh-frozen plasma and platelets. People who received a blood transfusion between 1977 and April 1985 – especially those who received multiple transfusions, such as those with hemophilia (a disease characterized by delayed clotting of the blood) – had a higher risk for exposure to HIV than those who have received transfusions since. The U.S. blood supply has been screened for HIV since 1985, nearly eliminating the risk of contracting HIV through the blood supply since that time.
  
  
• Other methods. In rare cases, HIV may be transmitted through organ or tissue transplants or unsterile dental or surgical equipment. There also is a small chance of infection during open-mouth kissing if there are bleeding cuts or sores in the mouth.