How often does the average American put on a clean pair of underwear? Probably not as often as most people would think.

In a recent survey, clothing retailer Tommy John asked 2,000 American men and women the truth about their habits -- and 45 percent said they'd worn the same pair of underwear for two days or longer.

Changing underwear once a day sounds like common sense, but according to Philip M. Tierno, Ph.D., clinical professor of microbiology and pathology at New York University, extending the time may not be as bad as it seems.

"Generally speaking, it’s not going to hurt you as much as your hygiene may hurt your reputation," Tierno told TODAY.

Everyone has naturally-occurring microorganisms in and on their bodies called a microbial flora, he said. Shedding these microorganisms is what causes clothes to smell bad after wearing them for an extended time.

While the fact that these microbes exist on the body may sound scary, the relationship between the host and organism is generally beneficial.

"Since they are your bacteria, unless you have a break in the skin or another conduit of entry into the body they won’t do you harm," Tierno said.

Underwear is the same as all other clothing in that extended wear leads to a build-up of the bacteria on the garment. The amount of accumulation depends on the activities the wearer does throughout the day. Sweating a lot on a hot day or while exercising will increase the population of bacteria.

So is it actually gross to wear underwear for more than one day? Maybe. But will it cause harm? Probably not.

"Two days would probably be very little detection unless you grossly contaminate your underwear," said Tierno. "If you talk about a week or two, now we’re getting into robust growth of organisms and clearly there will be odor."

Tierno explained that as long as the person clean well after using the restroom, there's no need to fear harm from extended undergarment wear.

What about underwear that have been in the rotation for a long time? The study reported 38 percent of American's don't know how long they've owned their oldest pair.

Once again, Tierno said most people shouldn't be overly concerned, as long as the underwear have been regularly washed after use.

"The only time you replace it is if it becomes gross or if it becomes functionally useless," he said.

The best way to wash underwear, Tierno added, is bleach on white undergarments and a color-safe hydrogen peroxide bleach for colored undergarments. He also recommended using the germicidal cycle, which usually has a UV light used to kill bacteria, for anyone with newer washers or dryers that have it.

What about underwear that have been in the rotation for a long time? The study reported 38 percent of American's don't know how long they've owned their oldest pair.

Once again, Tierno said most people shouldn't be overly concerned, as long as the underwear have been regularly washed after use.

"The only time you replace it is if it becomes gross or if it becomes functionally useless," he said.

The best way to wash underwear, Tierno added, is bleach on white undergarments and a color-safe hydrogen peroxide bleach for colored undergarments. He also recommended using the germicidal cycle, which usually has a UV light used to kill bacteria, for anyone with newer washers or dryers that have it.