What causes HIV/AIDS?
HIV is primarily spread when someone who’s already infected with the virus has unpro­tected vaginal or anal sex with a person who isn’t infected. Oral sex can also transmit the disease, but the chance of this is far less than with anal or vaginal contact. Overall, sexual transmis­sion of HIV accounts for the majority of all new infections. The virus can’t be spread through casual contact. The virus also cannot be spread through the sharing of toilet seats or food utensils.

In addition to being transmitted through semen or vaginal secretions, HIV can also be con­tracted through breast-feeding, from mother to child, or from infected blood—tainted needles or syringes shared by IV drug users being the most common route of transmission.

Blood transfusions are another way the virus can be spread, although the odds of being infected this way are extremely slim these days, due to more-thorough testing regimens for blood and donated organs/tissue.

What tests confirm a diagnosis of HIV/AIDS?
A conventional HIV test detects antibodies to HIV, using a sample of blood, urine, or saliva. But because it can take weeks for the immune system to produce these antibodies in response to an infection, there’s a “window period” between a potential exposure to HIV and test results. According to the CDC, 97% of people develop detectable antibodies to HIV within 3 months, but some may take as long as 6 months.

If there’s a strong suspicion of infection, a plasma HIV RNA test can detect HIV within 9 days of contracting the virus. If the result is positive, a second test, called a Western blot test, is ordered to confirm that a person has HIV. (Negative results, too, may need to be confirmed with a second test 3–6 months after suspected exposure.) Further tests—including those for CD4 count, viral load, and drug resistance—are used to determine which combination of drugs is likely to be most effective.

Early detection is crucial to preventing the spread of HIV: Findings released in 2011 from a multi-country study revealed that early testing and treatment can lower patients’ risk of transmitting HIV to an uninfected sexual partner by 96%.