Swimming in a lake could lead to a parasitic infection. The most common waterborne parasite is called giardia, and giardia actually afflicts about 3 million people a year.
Giardia irritates your intestinal tract, similar to the feeling of a stomach virus. To tell the difference, you can take a blood test or you can watch your poop. Since it affects your ability to absorb food correctly, if you have giardia, your poop will float. That's one of the clues to diagnosing an intestinal parasite. It will destroy the way that you actually digest food.
Giardia is found in a lot of water in the United States, especially water that hasn't been purified or filtered.
The most common reason people get tapeworms is from eating raw meat. That is the main reason humans began to cook their food.
Other parasites may have played huge roles in human history. Some people think that hookworm may have cost the South the Civil War. People who didn't wear shoes were particularly vulnerable to contracting a hookworm infection, sometimes called "Southern anemia," which left the host weakened. Because many Southerners who didn't wear shoes as children developed "Southern anemia," they grew up to be poor choices to be Confederate soldiers. Meanwhile many Northern children wore leather shoes growing up and never contracted hookworm and remained healthy.