system
Given the following post, respond as if you're commenting on Reddit:
human
Why are some viruses only transmitted by blood/sex and some only by saliva

Reading about HIV I got wondering *why* it's not transmitted by spit? What's special about blood and sex fluids that saliva lacks?

And then I realised that the cold & flu are transmitted by coughing/sneezing (presumably saliva) and I've never heard anyone get the flu from sex.

Why do some viruses get transferred by some methods while others get transferred by others?
gpt
Viruses rely on receptors on the outside of cells to recognize cells (and gain entry into them). Different cells have different receptors, and so different viruses recognize different types of cells. 

The same way that a lock pick will not get you into a door with a digital keypad, and a circuit analyzer will not get you into a door with a regular key lock.