How often should you wash your clothes?
A professor at the London School of hygiene and Tropical Medicine has found that many pathogens can remain on the surface of fabrics for long periods.
See also: Are you doing your laundry all wrong?
See also: How to remove armpit stains from your clothes
E-coli, example can remain on fabrics for hours, and salmonella can last up to 24 weeks on fabric.
Bear in mind, too, that a normal adult will shed around 500 million skin cells in a day, and that the average dirty underwear contains one-tenth of a gram of fecal matter, and you can probably guess the answer...