system
Given the following post, respond as if you're commenting on Reddit:
human
Why does HIV spread only via some body fluids? i.e. why does it spread via blood and breast milk but not via saliva and tears?

I'm especially intrigued by why breast milk but not saliva since both of them go through the digestive system of an uninfected person....
gpt
HIV is an enveloped virus. This means that it has a membrane that surrounds the capsid, which maintains the virus RNA. Viral envelopes facilitate the ability of the virus to infect cells; they also make the virus extremely sensitive to the environment, as slight changes in things like pH and tonicity (concentration of salts) can cause the envelope to rupture. Blood, breast milk, and semen contain components that 1)make them a buffer and therefore have a specific pH and 2) contain lots of dissolved salts, which maintains a specific tonicity. If you have ever seen someone receieve medicine through an IV, they use physiological saline because it has the proper concentration of salts and the proper pH. Tears and saliva do not have these properties; they are secretions, and do not contain components that regulate their pH/salt. Therefore, blood, semen and breast milk will facilitate the virus survival in the body, and also its transmission between organisms.
Interestingly, viruses that do not have an envelope, such as the polio virus, can survive for long times outside of the body. This is why water contaminated with polio is a serious health concern, but water contaminated with HIV, HSV, WNV, etc. is not.