you need to do a quarantine for 3-4 weeks. they can'nt even be in the same room together, or even the same house if you have central air conditioning, etc.
there are a lot of illnesses that especially feeder rats carry. I once walked into a pet store (petsmart that doesn't even SELL feeder rats), held a rat, then came home within 1 hour of holding the rat. about a week or two later, my rats had SDA and three died from it. I spend tons on vet bills for them... learned my lesson fast.
basically you have to watch out for sendai, SDA, and all sorts of other junk.. and just because they "look" healthy doesn't mean they're not carrying something passively. I had 14 rats with my SDA outbreak and a couple didn't even SHOW symptoms but were treated anyway with baytril and doxycycline.. it's just painful when they suddenly die from something prevenntable.....
just so you know for the future. hopefully the rat won't have anything though... and I've done it before, so I can't blame you, but also remember every rat you save, two more replace it. but I will say once in a while a little PEW just sticks in your heart ad you have to get them... I wound up with Beautiful Steven that way, as well as Harley and Wanda
2 rats, 2 mini rex bunnies, 2 cats, 1 prairie dog, 4 species breeding millipedes, 23 various different species tarantulas, 2 Heteropoda davidbowie huntsman spiders, 3 Liocheles australasiae scorps, 20+ Hogna helluo wolf spiderlings, etc.
spiders are addictive... and super easy to care for!