This doesn't make sense to me

[CadeOfRoots]

An animal is showing zero signs of infection, all green. but it still turns?
It just seems this way to make it impossible to actually cure them. What in the world do you do to cure something that has zero symptoms?

[Jewel the Espeon]

Its more complicated than that. The sick ones aren’t the ones that turn. The parasites mimic symptoms of an illness they witness in the other animals.
There should be a disk with recipes for anti-Parasite medication in one of the storage rooms. Find out what the non-infected animals have und pick (disease)-Toxin. You need to hit the mutant 3 times. It’s the only way to get a living parasite specimen as far as I know.

[Liveey]

Lets pretend we are entering an enclosure with four animals.

Testing shows one animal with all red symptoms (Plague)

However there are two animals acting sick! This means one is sick with the plague, and the other one will transform into the parasite.

Therefore you will need the cure for plague, and also the cure for an infected animal pretending to have the plague.

Shoot the infected animal before it turns and it will barf out the parasite, shoot the infected animal after it turns and you will need to shoot it three times.

You only need to shoot the sick animals with their respective cures once.