Does the baby have a milk belly or look well cared for?
If so I would leave the baby and check back in a little while to see if the mama returned for the baby. Hopefully she will soon feel safe enough to do that …… she was likely interrupted while removing her babies.
You need to find a nursing (pet) rattie mama with litter to carefully add the baby to …… so start looking for one immediately - one that you can buy along with all her babies.
Babies that young have almost no chance at survival if not fostered by a (pet) rattie mama.
The chance of the baby having a disease that your ratties could catch is so low that no one worries about it
Please see the Reference Thread for info on how to care for orphans …… until you can find a nursing mama with litter.
https://www.ratshackforum.com/threads/reference-thread-read-only.35894/
Baby will need to be kept warm (not too warm) and fed every 2 hours (incl all night) - with a new clean tiny paint brush, and will need to be stimulated after eating with a damp warm cotton ball so that it will pee and poop. Also
rescue formula and more info I posted is here :
https://www.ratshackforum.com/threads/infant-rat-uri-clicking.35697/#post-496055
Formulas are not good enough and most young babies often die after 5 days unless a foster mama is found ……. I have read that the formula for orphaned squirrels is the best, but people also feed organic soy infant formula