“would expose a susceptible creature like a guinea pig or rat to contaminated air making the filter successively finer until the animal was not infected”
I would think the opposite approach, starting with a very fine filter then enlarging it until the subject WAS infected would save you a lot of money on animal subjects...
My guess is they would use the laws of halves.
Try one, if it is fatal, go to half size. Try again.
If still fatal, halve it again. Then when you get one where the subject doesn’t get infected, you go halfway up to where you were before that.
Full disclosure-I have no idea if that REALLY is the way they did it back then. But the thing I read about Crichton said he actually spent a HUGE sum of money on equipment to have in the scenes, and consulted with the best people in the world on how it was done at that time.