Less infectious diseases like colds etc. that spread human-to-human seems likely given the isolation and lower population numbers. Where would the diseases come

Less infectious diseases like colds etc. that spread human-to-human seems likely given the isolation and lower population numbers. Where would the diseases come from? Of course, if they don't have access to antibiotics, they have other much worse kinds of sickness to deal with, and even their relatively simple lifestyle will be hard to maintain without hard work...