Researchers have shown that an average pair of underwear may have about a tenth of a gram of poop during a day. So, you put it into the washing machine along with other clothes and guess what? There will be about a 100 million E. coli and Staphylococcus aureus in the water.

Source: 6 reasons you must wash your underwear separately – Why you need to wash your underwear separately | The Times of India