[toufiqbarhamov]

The nightmare, of course, being a highly contagious, very deadly disease which let its victims infect others for weeks or months before falling ill themselves.

That’s a very good description of Variola Major, and one reason (along with lethality and disfiguring effects) that it was so terrifying.

[ams6110]

HIV/AIDS also.

[thaumasiotes]

HIV isn't very contagious. That is why dying of AIDS was a strong indicator that you were gay. If it had been able to spread easily, everyone would have had it rather than just a tiny subpopulation.