Was reading up on Ivermectin just now.
It's used in both humans and animals for the same reason, it kills parasites.
A parasite has to have a host. It can't exist without one. That why you usually get them from eating some protein.
A virus doesn't need a host - it can survive in the breath from a cough or sneeze or on a surface that you touch.
Parasite medicine to treat a virus?
Parasites can be killed. Viruses can't, only defended against.
How the hell did this start catching on?
https://abc7news.com/ivermectin-covid-humans-side-effects-treatment/10994919/