About 17 million pairs of feet move through Lindbergh Field Airport in San Diego every year. Each passenger is asked to take off their shoes at security. Some are clean, some are dirty, and some carry diseases, 10News reported.

"If there are individuals walking barefoot or with socks, they will transmit to another person walking over the same spot," lower leg desease expert Dr. Daniel Lee said.

What do socked or barefooted passengers leave behind? 10News used petri dishes to gather samples where passengers picked up their carry-ons off the conveyer belt at the security checkpoint.

After sampling in San Diego, a 10News employee headed to Las Vegas and Phoenix -- two popular destinations for San Diegans.

Investigative reporter Marti Emerald gave the samples to Quadrants Scientific in Rancho Bernardo.

The lab identified a mold from McCarron Airport in Las Vegas as trichophyston, which causes ringworm and favus, a nasty disease of the scalp.

The next test was from Sky Harbor Airport in Phoenix, where samples were lifted to test for bacteria.