Germs, like you and I, enjoy the warm and slightly moist embrace of a bath towel. (Hey, if it isn't moist when you first put it on, it gets moist.) But these germs are just here to cuddle; normal use of a bath towel is not likely to make you sick.
Many of us were momentarily terrified by a tweet that likened an unwashed towel to washing one's face in the toilet. The source: An Apartment Therapy article entitled "You're Definitely Not Washing Your Bath Towels Enough".
The article was inspired by a survey whose results implied that we should all be doing more laundry. The survey was sponsored by a company that sells washing machines.
— Bim Adewunmi (@bimadew) June 4, 2018
But wait a second. We don't have any evidence that people are getting sick from their own bath towels. The toilet-washing quip came from a scientist originally quoted in this Time article, which also quotes other scientists saying, "Our bodies are adapted to being able to live in this environment with all of these microbes around," and, "As long as it's drying completely between use, there's almost no chance of passing bacteria from one person to another."
The truth is, there are bacteria on our towels because there are bacteria everywhere. "After all, where did the microbes on the towel come from, mostly? You and me," says Mark Martin of the University of Puget Sound. (I assume he is referring to his and my towels separately. I do not share towels with microbiologists I meet on the internet.)
Studies finding E. coli or "coliform" bacteria, Martin points out, are usually just looking for bacteria that are gram-negative and can metabolise lactose. These bacteria aren't necessarily bad for us, and they don't necessarily come from faecal contamination.
You'd have to do extra tests to confirm that bacteria you find this way are putting people at risk, plus you'd have to figure out whether there are enough of those bacteria present to make a person sick.
Martin washes his towels once a week, as does fellow microbiologist Jack Gilbert of the University of Chicago. Washing any more often, he says, "wouldn't do much unless people had gut infections and you were rubbing [the towel] in your mouth and eyes." So don't do that.