Causes and Transmission of HIV

Instructor: Adrianne Baron

Adrianne has taught high school and college biology and has a master's degree in cancer biology.

You have likely heard all kinds of information about HIV, but may still have questions about exactly how you get it. This lesson will describe how HIV is transmitted from person to person.


You have heard about this very scary virus for years. Over time you've also heard a lot of conflicting information about HIV or the human immunodeficiency virus. This virus is one that infects the body and attacks the immune system. It continues to attack until the immune system is completely unable to function during the last stage of the infection, which we call AIDS or acquired immune deficiency syndrome.

If you are like most people, you want to understand exactly how this virus is passed from person to person, or is transmitted.

Transmission Routes

HIV is carried in certain fluids in the body such as semen, blood, vaginal secretions, and breast milk. The ways that HIV can be passed from one person to another are based on the bodily fluids that carry the virus. Despite many misconceptions, it cannot be passed through skin-to-skin contact with intact skin or via saliva.

The virus is transmitted by sexual contact through the exchange of semen and/or vaginal secretions. There is a difference in how easily the virus is passed depending on the type of sex. The easiest way to pass the virus during sexual contact is from one man to another man who is receiving anal sex. The next easiest way is through vaginal sex from a man to a woman. It is also passed, but less easily, from the man receiving anal sex to the man giving it and from the woman to the man during vaginal sex. HIV may also be passed through oral sex in either direction, as well as from woman to woman during vaginal interactions, but it is not as easily transmitted this way.

Sharing needles is a mode of HIV transmission
Picture of two people with one needle

The next most common way for HIV to be transmitted is through sharing needles that are used in taking drugs. Blood from one person is likely to get on the needle when it is used. Anyone who uses the needle after that will get some of that person's blood in them. If the first person to use the needle has HIV, then it can be transmitted to everyone who uses the needle after them.

Sadly, there are also times when babies are born with HIV because it is transmitted to them during fetal development inside their mother. This doesn't occur very often anymore because the mom can be treated, ensuring that the chances of passing HIV to her unborn child are slim.

The other possible way that a mom might transmit HIV it to a baby is through breastfeeding. Transmission through this route is also rare since the moms are normally advised to not breastfeed if they are HIV-positive or they are given medications if they do choose to breastfeed.

HIV can be transmitted to healthcare workers by an accidental needle stick
Picture of a healthcare worker giving an injection

The last possibility (which has also decreased in recent years) is through an accidental needle stick. This method of transmission is likely to involve a healthcare worker or caregiver. Transmission happens when working with an HIV-positive individual. If a needle used for an injection or blood draw accidentally sticks the healthcare worker or caregiver after being used, then there is a chance that the virus can be transmitted.

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