I used to have a problem with one of my cats which I worked out was that every time she came back from the cattery I used, a few days later she was sneezing & snotting all over the place, yes not very pleasant is it?
I worked out that it had to be the cattery & when I used another place the problem stopped, but not until after I worked through the idea that she got the infection from the cattery, & not from coming back home.
But each time I did take her to the Vet & despite moaning about the price of fees etc., I could not put her health at risk.
I'm sure there must be some kind of animal welfare place that will help if you can't afford the fees.