In Philip Tierno's mind, a moist towel is practically a living organism.
"A damp towel is growing," Tierno, a microbiologist at the New York University School of Medicine, told Business Insider last year.
Towels serve as ideal breeding grounds for germs because they contain many of the requirements for microbial life: water, warm temperatures, and plenty of oxygen. Where there is moisture, there are microbes.
Because of this, Tierno says you should only use a towel three times before tossing it in the wash -- and that's assuming it gets completely dry after each use.
A good indicator that a towel has remained damp for too long is the smell.
"If there is odor coming from the towel, wherever there is odor, there are microbes growing, so it should be washed," Tierno said.
By drying yourself with a towel coated in bacteria, you're likely transferring that bacteria onto your skin, potentially undoing the main purpose of a shower.
And if you're sharing a towel with other people, you could come into contact with organisms that your body isn't used to dealing with -- such as Staphylococcus aureus, Tierno said, "which may give rise to a boil, or a pimple, or an infection."
It's important to keep in mind, however, that not all microbes are bad for us. In fact, many of them are harmless and some may even be beneficial in the long run. According to an idea called the hygiene hypothesis, exposure to germs and certain infections -- especially at a young age -- may actually help prime the immune system so it can defeat foreign microbes more easily in the future.
Still, no one likes a smelly towel -- especially when that odor means the cloth is teeming with microbial life.
"The idea is to be prudent and to be aware," Tierno said.
Julia Calderone wrote an older version of this post.