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By Medical Author: Pallavi Uttekar

By Medical Reviewer: Shaziya Allarakha

Numerous studies have been conducted to investigate the possible ways through which human immunodeficiency virus (HIV) spreads from one person to another. These studies proved that HIV cannot be transmitted through surfaces such as toilet seats, chairs, doorknobs, drinking glasses and bedsheets. The virus cannot survive outside a human host; hence, transmission through air, water (swimming pools), insect bite or casual contacts such as handshake, hug or touch is not possible. No case of passing the infection through scratching or dry blood has been ever documented. Although trace amounts of HIV are found in saliva, sweat, human waste and tears, the viral load is too low to infect another person.

Laboratory-grown HIV when placed on a surface tends to dry over the next few hours and becomes noninfective by more than 90 percent, causing little to no risk of infecting another person.

How well does HIV survive outside the body?

Human immunodeficiency virus (HIV) does not survive well outside the body. HIV transmission is only possible through certain body fluids such as blood, semen, vaginal fluid, rectal fluid and breast milk. Intact skin is a 100 percent barrier to virus transmission. A needle prick that causes the blood to ooze, sexual contact, blood transfusion (which is extremely unlikely because all blood donors are screened for HIV and other blood-borne infections) or infection from a mother to her child during delivery are the ways through which HIV can be transmitted. There have been rare reports of HIV transmission through human bites deep enough to draw blood.

Many laboratories have used artificially high concentrations of laboratory-grown HIV to study how long and to what extent HIV survives on surfaces. The Centers for Disease Control and Prevention (CDC) have reported that even fluids with the highest concentrations of HIV dries out over the next few hours and become noninfective to the tune of 90 to 99 percent. Thus, drying of the HIV-infected human blood or other body fluids reduces the theoretical risk of environmental transmission to essentially zero. HIV is unable to reproduce or divide outside its living host, except under strictly controlled laboratory conditions.

What are the potential routes of the spread of HIV?

For some exposures, although transmission is biologically possible, the risk is so low that it is not possible to put a precise number on it. This includes deep kissing or indulging in oral sex. 

Main routes for HIV transmission are as follows.

• Sexual route: Receptive anal sex (receiving the penis into the anus) is the riskiest sexual behavior associated with HIV transmission with 1.4 percent risk. Insertive anal sex (the act of inserting persis into the anus) carries 0.06 to 0.62 percent risk. Receptive vaginal sex (receiving the penis in the vagina) can pose a risk of 0.08 percent, whereas insertive vaginal sex (inserting the penis into the vagina) poses an estimated risk of 0.04 percent. Remember that transmission can occur after just one exposure. You are at a higher risk of infection if:

• You already have a sexually transmitted infection (STI).

• You engage in activities that can cause tearing and inflammation such as rough sex, longer sex, douching, enemas before anal sex and tooth brushing, flossing or dental work before oral sex.

• Your partner has a high viral load during sex. This is particularly observed during the first few weeks of getting infected or when an infected person does not take medications for HIV treatment. Proper treatment not only improves health but also lowers the viral load, thereby reducing the risk of infection transmission.

For every 10-fold increase in viral load in your partner, your risk of HIV infection increases by two to three times. Therefore, unprotected sex with a person who has acute HIV infection increases transmission risk to up to two percent (equivalent of one transmission per 50 exposures) for receptive vaginal sex and over 20 percent (equivalent to one transmission per five exposures) for receptive anal sex.

• Blood-borne route: Because international health regulations require all blood products, transplanted organs or tissues to be rigorously screened for numerous viral or bacterial contaminations before transfusion/transplant, the risk of HIV through blood transfusion or organ transplants is negligible. The risk of HIV infection through blood transfusion is estimated conservatively to be 1 in 1.5 million. Regular dialysis presents a low risk of HIV, although occasional cases have been reported due to improperly reprocessed patient-care equipment and infected access needles.

• Shared needles: Drug users who share needles while injecting the drug can get infected with HIV. HIV can transmit by blood inside the bore (the hole down the middle) of used needles, or if there was any blood inside the syringe. Sharing needles or syringes during drug use carries around 0.67 percent risk of HIV transmission. Sharing water to clean injecting equipment, reusing containers to dissolve drugs and reusing filters can also transmit HIV, although exact risk is unknown.

• Needlestick injuries: The risk of HIV transmission from a needlestick involving HIV-containing blood is estimated to be 0.23 percent or a little over 1 in 500. The risk of HIV transmission during tattoos and piercings through infected needles is a possibility, although exact risk is unknown.

• From infected mothers to their babies: Women with HIV may pass the virus to their babies during pregnancy or delivery in about 15 to 25 percent of cases. Risks of transmission through breast milk are about 5 to 20 percent. In the United States, mothers with HIV are advised not to breastfeed their infants regardless of their antiretroviral therapy (ART) status and maternal viral load to protect their child.