Your office coffee mug you use for just about every beverage could cause you to fall ill, says an expert. People rarely take their office coffee mug home for washing and many times they scrub it at the office with sponges and brushes that have been used by everyone, making a home for bacteria. In fact, Charles Gerba, Ph.D., a professor of environmental microbiology at the University of Arizona, says 20 percent of mugs carry fecal bacteria.