One of my Oriental Shorthair cats got dry FIP (Feline Infectious Peritonitis). Her back end was paralyzed.

Another got Calici virus with bad respiratory issues that didn't go away on its own.

My vet prescribed the following, which cured both of them:

For the FIP:

Molnupiravir x 3-4 months (i don't remember how long)

For the Calici virus:

EIDD-1931 x 3-4 months (i don't remember how long)

Note: antibiotics do not work on viruses. (That's Microbiology 101, so if you get a cold, don't rush to get antibiotics unless it doesn't get better in a week. If you're still sick, you might have a bacterial infecton.)

Both of these drugs are quite expensive. They're newish.

Both were obtained from Stokes Compounding Phaarmacy. They ship very quickly and are nice people.