If enough parasites manage to get through, they cause katayama fever. Left untreated, schistosomiasis can lead to cirrhosis of the liver, bladder cancer and anemia. Research your destination and if there are reports of schistosomiasis, stay out of freshwater lakes and rivers. Another disease caused by freshwater parasites is Leptospirosis. Recent cases have been reported in Hawaii and Central America.
Leptospirosis can cause meningitis and liver disease as well as generalized fevers and aches.
While there are no vaccines to prevent these types of parasitic infections, they are easily treated when properly diagnosed. Many parasites may not make you sick until weeks or even months after initial infection. That's why it's important to pay particular attention to any illnesses that you may have after traveling. Use common sense. If it's flu season, and everyone around you has the same symptoms, then you're probably justified in explaining away your cough, fever, headache and malaise.