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Let us be thankful for winter, perhaps the greatest public health measure ever created.

It's hard to have perspective while we're freezing. I myself, when I get off the plane in Florida to visit my aunt in December, frequently wonder why I live in purgatory.

But winter, we should all keep in mind, beats back a great many infectious diseases that plague the bulk of the human population. The cold kills off both pathogens and their vectors -- the animals and insects that act as hosts to the viruses, bacteria and parasites that make us sick.

For example, cold weather keeps down mosquito populations. The mosquito that carries dengue fever, Aedes aegypti, does not survive wintertime, and can only live as far north as Memphis.

As such, we in this region are spared "breakbone fever," as it is called due to its painfulness. While the disease is not fatal, it is unpleasant, and populations from South America up through the Caribbean are affected. We, here in our winter purgatory, are spared.

The mosquito also carries yellow fever, a similar but much more serious disease that can kill up to 20 percent of those it infects. The disease usually rolls along through tropical and subtropical regions, but it became a public health problem in the 1700s and 1800s in American port cities, even Northern ones, as commerce brought the infection to populations that had never been exposed to it. Yellow fever was, in fact, the disease that most contributed the impetus for forming the practice of quarantine and for creating the first hospitals in this country.

Public health measures certainly help keep that disease under control, but it won't have a chance up here, until either the climate warms up quite a bit, or it finds a way to live in other mosquitoes.

Textbooks of pathology are full of gruesome descriptions of parasites that affect only tropical regions: intestinal worms, blood flukes -- that sort of thing. They just can't survive up here.

Let's talk about a relatively palatable one: Trypanosoma cruzi, the single-celled organism that causes Chaga's disease. Chronic infection with the parasite is a major cause of death in Latin America. It causes thinning of the walls of the heart and leads to heart failure and irregular rhythms. It can also cause dilation of the esophagus and colon, causing difficulty swallowing and constipation.

The malaria parasite, Plasmodium, has been endemic in the United States, but is rare here now. Mostly public health measures are responsible, but colder weather plays a part in suppressing transmission and replication of the parasite.

The only real parasites we have to worry about in New York are Giardia and pinworm. Giardiasis, also known as "beaver fever" after a common carrier of the disease, is a single-celled organism that, when consumed in contaminated water (be careful in Allegany State Park!), reproduces in the intestines and causes a gassy diarrhea. It's usually just a nuisance and responds to an antibiotic quite well. Pinworms are one cause of what we in the medical profession refer to technically as "itchy butt," and also are easily eradicated with medication.

Now winter obviously has some very serious down sides. For example, people are far more likely to die of cold exposure than heat exposure. Also, during snowfalls and cold weather, heart attack rates go up, and obviously there are injuries due to falling. It brings a certain humility and introspection when one reflects that, as Garrison Keillor has said, every winter Mother Nature tries to kill us.

But let's see things in perspective. My children aren't going to die of cerebral malaria. When my patient pees blood, it's a urinary tract infection and not a blood fluke living in the vessels around his bladder. The average U.S. senior citizen may have eye problems due to diabetes, but won't get River Blindness.

In this place, Nature is moody, but it does us the favor of driving away disease.

MIKE MERRILL, M.D., is a resident in internal medicine and preventive medicine at Buffalo-area hospitals. He also works in medical informatics for a health insurance provider. His first-person accounts of the medical profession appear regularly in Viewpoints.

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