"If you are in a river or in a lake that's contaminated from other humans or animals, that's how you catch E. coli," he said.
E. coli may get more attention, but Tzipori said the chances are much greater of a person contracting Campylobacter because many animals can get it, and many animals can pass it to humans.
"It's the most common source of gastroenteritis in the United States," Tzipori said. "But it's not very serious, so you tend to hear less about it."
Also common are Giardia and Cryptosporidium, which sickened 400,000 people in Milwaukee in 1994.
"It [Cryptosporidium] is found in pools, and it's very resistant. It hangs around for a long time," said Tzipori, who added it would only take a few Cryptosporidium to make a person sick, in contrast to the relatively more E. coli needed to cause an infection.
Despite the prevalence of these diseases, experts say it should be safe to swim -- not drink -- in the great outdoors.
"If you were swimming in the water and you weren't ingesting the water, you should be OK," Wade said.
Summer picnics are marked by that iconic image of ants marching across a red-checkered cloth as helpless picnickers look on. Ants, flies and other insects are an accepted part of eating outdoors, particularly in the summertime, but they are not innocuous.
"The thought of a fly getting on the dog poo-poo and getting on your hamburger is not a very nice thought," Kentucky's Bastin said. But she noted that the overall health risks from insects landing on food were minimal, especially compared to leaving foods unrefrigerated for long periods of time.
The "danger zone" in which bacteria find it easy to grow, given enough food and moisture, is between 40 and 140 degrees. The longest stretch that food should be left above 40 degrees is about two hours.
And Bastin pointed out that time spent in the grocery store, driving home or to a picnic site and preparing and setting up a meal counts as part of that two-hour window.
"[These bacteria] are pretty common, so we're depending on cooking and handling food properly to keep us from getting sick," Bastin said.
While bowls of potato salad and cole slaw left out in the summer heat are sources of potential food poisoning, mayonnaise may have unfairly developed a bad rap.
Contrary to popular theory, mayonnaise alone is not a breeding ground for salmonella or E. coli. Commercially prepared mayonnaise is made with pasteurized eggs, effectively limiting the risk of salmonella. And because mayonnaise is made with lemon juice or vinegar, bacteria don't naturally find it easy to grow in such an acidic, low pH environment.
"But the minute you cut the baloney slice or the potatoes in potato salad and put the knife into the mayonnaise, it changes that pH," Bastin said. "When you combine [those foods] with mayonnaise, the pH goes up high enough so the bacteria will grow."
And often foods using mayonnaise are meant to be served cold. Leaving such foods out too long increases their temperature and encourages bacterial growth.