I used to be a lab tech back in the day. In one of my jobs, we were experimenting with concocting a usable vaccine for young children, and “naked” mice were our subjects. They had no immune systems to speak of. When we got to the part where we were injecting them with the bacteria, we were in completely sterile rooms and were completely gowned, masked, and gloved in order to hopefully infect our mouse “patients” with the only thing we wanted them to be infected with, Haemophilus influenza type B, IIRC. Our masks and garb were worn to protect our “patients” from getting anything from us, not to protect us from them or the HiB.
To every rule there is an exception.