system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does Truvada (PrEP) help prevent HIV?
gpt
Truvada is a reverse transcriptase inhibitor.

Those sounds like complicated words so let me explain them.

Viruses work by injecting their DNA into a host cell. This DNA will use the cell’s machinery to make copies of the virus itself, until the point where it makes so much virus it exhausts the cell’s machinery, killing and rupturing the cell at which point all the fresh virus is released into the body to infect more cells.

There are a number of ways a virus can do this. There are also a number of ways the body has built systems to protect itself from viruses.

The goal of a pre exposure treatment is to stop the virus from injecting its DNA into the cells in the first place. Our bodies are pretty good at getting rid of the actual viruses themselves, but we have no good way of getting rid of virus DNA hiding inside cells making copies of the virus. The DNA could even hide inside cells for decades until at one point it suddenly starts printing more virus. 

So PREP is a reverse transcriptase inhibitor. HIV is a sneaky virus in the sense that it uses RNA, which it then copies into DNA when injecting into cells. Most viruses don’t work this way, they either contain DNA or RNA which they inject into cells by itself, humans naturally have pretty decent defenses against virus RNA inside cells and virus DNA outside of cells, but HIV prevents this entirely by being RNA outside the cell and DNA in the cell. This is partly why HIV is so devastating. 

The chemical responsible for changing RNA into DNA is known as reverse transcriptase. PREP contains chemicals that inhibit (attack) this reverse transcriptase, preventing virus from injecting into cells in the first place, buying time for the body to kill off the virus lingering outside of cells.

This is similar to how modern HIV drugs work in the first place, the issue is that once you have a few cells infected with HIV, the HIV will likely hide in some cells for the rest of your life, therefore drugs must be taken for life to prevent this. This is why HIV these days is no longer a death sentence, but it’s best to prevent it in the first place. Therefore PREP floods the body with chemicals to attack HIV before the HIV has a chance of being in the body in the first place.

PREP actually has two reverse transcriptase inhibitors that work in different ways. The problem with having one is that there is a chance that HIV will mutate to prevent the operation of one inhibitor, the chance that it will evolve to prevent two at the same time is very small, if it evolves to defeat one, the other takes care of it.