You can make the case for whatever case you are trying to make, without pushing back on this quite specific extremely correct point. When people meet. disease spreads. If it doesn't spread when they breathe on one another, it will spread when they touch. If it doesn't spread when they touch, it will spread when they fuck. If it doesn't spread when they fuck - phew, crikey, this disease is useless indeed, and natural selection will see it off quickly. Meanwhile our protagonists now have flu, norovirus and crabs.