How frequently should you clean your coffee mug?

People adore their coffee, but most don't show the same love for its loyal vessel, the coffee cup. Office workers are especially prone to this neglect, simply rinsing and reusing their mugs day after day. Or worse, they just discard the old coffee and refill it with a fresh brew.

While you might think the mug is fine with such treatment (after all, hot water kills germs, and you're the only one using it), the reality is even more surprising than the caffeine itself. A mostly empty mug, left alone, can quickly grow some quite alarming things.

"All types of germs and bacteria can live on and inside your cup, particularly if you're in a shared space," explains Melissa Maker, host of the YouTube Channel Clean My Space in an email interview. Maker points out that most bacteria are harmless, but some can be quite harmful. "Cold and flu germs often spread easily and can end up on or in your cup. Additionally, germs from your mouth and hands also transfer to your cup," she adds. "The warmer and more humid the environment, the more inviting it is for bacteria to thrive."

Indeed, the list of potential bacteria is intimidating. "Streptococci, staphylococci, and various oral bacteria, some of which may be harmful pathogens, can be present in unwashed cups," says Dr. Terry D. Connell, professor of microbiology and immunology at the University at Buffalo, via email. "And, of course, if someone else drinks from your cup, bacteria from their mouth can be transferred into it."

Office spaces are particularly prone to dirty coffee cups. Dr. Charles Gerba, a professor of microbiology at the University of Arizona, published research findings in 1997 that still shock us today. His team studied bacteria in office coffee cups and coffee prep areas and found that 41 percent of the cups tested were contaminated with coliform bacteria – commonly known as fecal bacteria! Although these bacteria are usually harmless, they can be dangerous, particularly for people with weakened immune systems. The surprising part? Dr. Gerba's team discovered that a small office cup washer (similar to a mini dishwasher) was all it took to eliminate these and other types of bacteria.

The problem is, many people believe that a simple rinse and wipe is sufficient to prevent germs; however, this method is not nearly enough to deal with such strong foes. "Bacteria can stick to the surface of the cup," Connell explains, adding, "Washing with soap is crucial to removing those bacteria."

That's why public health officials in England issued a warning back in January for reusable cup users to thoroughly clean their cups after every single drink to prevent the spread of illness. This was in response to the rising use of reusable cups since the country introduced a "latte" tax on disposable cups.

Proper cleaning becomes even more crucial if your coffee has been mixed with cream or sugar. "Both of these ingredients provide nutrients for bacteria to grow. While rinsing with water will remove most of the sugar, the oils and proteins found in milk or cream require soap to be properly cleaned," Connell explains.

Here are simple steps to reduce the germs in your coffee mug: