Although I also write about pinworms, round worms, tape worms, and a variety of other parasites, probably the most familiar of all the parasites discussed on this site is trichinosis.
The trichinella worm (which causes trichinosis) is a tiny parasite about 1 mm (1/25 of an inch) long that lives in the lining of the intestine. The larvae of the organism grow a tough coating that protects them from stomach acid. In fact, the process of digestion in the stomach releases them so they can attach themselves in the colon.
Trichinella grow in the human body, but they do not multiply there. The disease can be acquired from eating undercooked pork, black bear, polar bear, wild boar, warthogs, or walrus.
Serious damage to internal organs only occurs if the worms reach maturity without leaving the intestinal tract after they develop the protective coating. After about a year the larvae calcify and can then interfere with circulation.
Hormonal changes associated with trichinosis can cause pregnant women to have a miscarriage or deliver a premature baby. The worms can also lodge in the heart, lungs, and central nervous system, but this only happens in about 1 in 300 cases. Symptoms such as headache, hives, and fever, lasting 3 days to 6 weeks, are somewhat more common, but most people who get trichinosis have no symptoms at all.
While trichinella are growing in their human host, they cause problems with nutrition, but not in the straightforward way associated with, say, tapeworms.
Trichinosis differs from other parasitic infections by interfering with the transfer of information between the brain and gut. In trichinosis, hormonal signals associated with a full stomach are sent to the brain earlier and earlier. Only when the immune system's T-cells become depleted by malnutrition does the trichinosis worm stop intefering with these hormones. Once the immune system is weak, then the parasite allows the stomach to signal the brain that it needs more food.
The only country you can absolutely avoid trichinosis is Australia. Since 2005, cases of trichinosis have been reported in Brazil, Canada, China, France, Italy (the island of Sardinia), Latvia, and Poland, although most cases were contracted from eating undercooked game. It's possible that trichinosis is found in many other countries but just not reported.
What do you do if you have trichinosis? In a phrase, not a lot. Doctors usually only treat severe cases. Assuming you are not cannibalized on your visit to the highlands of some tropical island, you will not infect other people.
How do you avoid trichinosis? In the case of fresh pork (bear, boar, walrus, or warthog), it's essential to cook the meat to a temperature of 70 degrees C (about 170 F) so there's no pink at all inside.
Freezing for at least 4 weeks also kills the parasite, as do pickling and smoking. That's how the Rohschenken (raw hams) of the German-speaking world are free of the parasite.