thats interesting something to know about regarding any contagion
Fostering can be daunting, especially since it more typically entails fostering high risk cats, but in this particular case, it is because the shelter is running out of space rather than because of cats who are high risk medically. So you have the opportunity to save lives here with minimal risk to you and your own cats.
Firstly, make a plan. Where and how many cats are you able to foster temporarily? I use my guest bedroom when I foster kittens and basically stripped it of anything dangerous or difficult to clean. I made an enclosed area with a plastic sheeting bottom (drop cloth) covered with washable bedding material (old towels, blankets, etc.). I use separate litter boxes and food bowls. Shelters often have extra basic supplies like these that you can bring home with the cat(s).
Secondly, make sure both your own cats as well as cats you foster have been vaccinated (at least for FVRCP and rabies), dewormed, and defleaded, and treated for ear mites if necessary and that body temperature is in normal range. Shelters generally do this will do this as part of a routine work up on incoming cats anyway. See if they routinely test for FeLv, and panleukopenia or if they have any outbreak of contagious disease recently.
Kitten fostering is more risky than adult cats because kittens are more suseptable to disease due to underdeveloped immune systems, and that again becomes the case with older cat (over 10 or so) due to decline immune system vitality that comes with age. An immunized cat over a year old and under 10 years old that currently appears healthy is at minimal risk for significant contagious disease. I'm excluding minor URI, parasitic infection and fungal infection here because while a nuisance, they are easily treatable, especially in an otherwise healthy cat.
Of course you already know my recommendation......go for it! But I may be a bit biased :-) Fostering can be a very rewarding way to help cats, help your shelter, and bring a lot of joy and satisfaction to you. I literally burst with pride and joy each time a cat I have fostered is adopted into a forever home knowing that I have had a part in that. I know that I can't feasibly have 100 cats of my own, but fostering and volunteering at the shelter lets me do that in a feasible way, so for me it is a win/win situation all the way around.
Please let us know what you decide, and I would be more than happy to answer any questions or help in anyway I can based on my own experiences with fostering. I can't think of a better way to celebrate the true spirit of Christmas than to foster a cat that otherwise might be euthanized for "no more room at the inn" at your shelter.
Great question, Opus. Although I would dearly love to foster a kitty, I have the same concern as you - what kind of disease might that cat be carrying since of course, you want to protect your own. With all I've learned here, I hesitate to even pet a cat on the street while walking for fear they might have ringworm or any number of things.