

two mice passed within 4 days of each other. what could have happened?
background: i got three mice from a feeder bin a few months ago. Bonnie (light orange, bottom left second pic). Maddie (siamese, top right second pic). Alexis (not pictured). about a month ago, alexis suddenly got sick. she just became lethargic and weak, didn’t eat, looked like she was in pain. she was gone by the next day, before i could even take her to the vet. a little while passed and i adopted three rescue mice, a little older than bonnie and maddie, to be companions. Ginny (orange, bottom right second pic). May (black, both pics) and Hayley (grey, both pics).
less than a week ago, Ginny suddenly got lethargic and sickly. she was still new and therefore shy, it was normal to not see her at every feeding time, so by the time i went looking for her she was in rough shape. i tried to get some food into her and keep her comfortable, but when i woke up the next morning she was gone.
Yesterday, May got lethargic. she looked the same as ginny. i kept an eye on her overnight but this morning she was looking the same, so i separated her along with Hayley. just now she passed away too. four days apart.
Does this sound like any known mouse illness? my leading theory is that the feeder bin mice have some dormant illness, it got to Alexis because her immune system was weaker (i think she was super inbred) and then they passed it to my new mice upon introduction. but is that even a thing that happens? has anyone else experienced this? i really just want to make sure my remaining three mice are going to be okay.