At last, you've made it: You're meditating on a forest log; you're plopped on a deck chair in the Adirondacks, overlooking the lake; you're kicking back by the barbecue. Just you, the sun, the breeze—and 5 billion years of microbial evolution. With the American back yard and playground expanding to include areas once considered rural, the odds are increasing that your time outdoors will bring you into contact with some form of wildlife—beavers, raccoons, deer—and the microscopic pathogens they carry.

Beyond the range of urban water-treatment systems lurks Giardia lamblia, the one-celled wonder responsible for giardiasis. Giardia is an intestinal parasite that sets up shop in various forest animals and enters the water supply through their feces. You drink the water, you get the parasite. Two decades ago, giardiasis was relatively rare in the States; now it's one of our most common waterborne diseases, carried to remote lakes and mountain streams by beavers, muskrats, and, the biggest culprit, campers who don't properly bury their fecal waste.

Once it's in your small intestine, Giardia lamblia gets things moving: cramps, flatulence, and diarrhea are common symptoms. For a parasite transmitted through excreta, this cycle works out nicely; for you, it's not so good. Symptoms usually persist for a week or two, confining your outdoor vacation to within racing distance of a rest room. Drugs are readily available, but American doctors don't always think to test for giardiasis—be sure to inform yours of your travels.

Far more fearsome than giardiasis, although statistically a smaller threat, is rabies, which in recent years has seen a resurgence in the Northeast and Midwest. There are several strains of rabies, each linked mainly (but not exclusively) to a particular species and region of the country. In the western and central United States, the virus is most commonly spread by skunks; in western Texas and Arizona, by gray foxes. East Coast health officials have been closely monitoring a recent outbreak of raccoon rabies. In 1977, a handful of rabid raccoons found their way from Florida into West Virginia—released there, it seems, by unsuspecting hunters keen on expanding the range of their quarry. Rabid raccoons then began appearing farther north: in New York in 1990; Rhode Island, Vermont, and Maine in 1994. Raccoon rabies had reached Ohio by 1997; it's expected to continue spreading westward.

The rabies virus is transmitted through an animal's saliva, usually via bites, and is brutally efficient: once in the central nervous system, it makes its way to the brain cells, which soon deteriorate. Victims become paralyzed, have difficulty swallowing, and sometimes even foam at the mouth like the affected animals. Treatment involves a monthlong series of immunizations—but by the time symptoms appear, in anywhere from 10 days to a year or more, an excruciating death is unavoidable.

Fortunately, the number of people who die of rabies in the United States is extremely small. As Charles Trimarchi, director of the rabies lab at the New York State Department of Health, points out, "the only people to get rabies are those who don't know they've been exposed." For instance, bats don't usually carry rabies, certainly far less frequently than raccoons and skunks do, but their bite can go undetected—bat bites are small—especially if you're asleep when they dig in. Indeed, of the 22 rabies-related fatalities this decade, 20 were thought to have been caused by bat bites. If you wake up with a bat in your room, Trimarchi says, or if you find one in a room where a child has been sleeping, it's worth capturing the animal and having it tested. Don't touch the bat yourself: put on some leather gloves, wait until it lands, then approach it slowly and place a box or coffee can over it. Slide a piece of cardboard underneath to trap the bat, tape the sides together, then hand the whole package over to the health authorities.

May, June, and July are prime months for encountering other unpleasant critters, notably ticks of the genus Ixodes, which are responsible for transmitting the spirochete (a corkscrew-shaped bacterium) that causes Lyme disease. Ever since Lyme disease was first recognized in the United States in 1975, as a mysterious outbreak of arthritis near Lyme, Connecticut, it has been the subject of news reports and scientific studies. Since then it has expanded its range considerably, encompassing the Northeast from Massachusetts to Maryland, the north-central states, especially Wisconsin and Minnesota, and the West Coast, particularly northern California.

Lyme ticks are an opportunistic bunch: they hang out on the tips of shrubs and grass blades, waiting for a mammal—a deer, a dog, you—to brush past. Their bloodsucking life cycle takes them from small animals, typically mice, to big ones, usually deer. With deer populations at an all-time high, the odds of the casual hiker or weekend-home owner coming across a Lyme tick are considerable.

Last December, after extensive testing, the Food & Drug Administration approved and licensed the first vaccine for Lyme disease. Although the Centers for Disease Control and Prevention doesn't recommend the vaccine for people at low risk, it does encourage those living in affected counties who frequently venture into tick-infested territory to "consider using it." Three injections are required: two in the winter months (before tick season) and the third a year later. Dr. David Dennis, a CDC researcher who helped develop the vaccine, notes that it's safe but doesn't always work. Studies found it 50 percent effective after the first two injections and about 80 percent effective after three. Whether later booster shots are required is still unknown. The bottom line?"We strongly recommend that people continue personal protective measures against tick bites," Dennis says. "You shouldn't be complacent about tick exposures."

  • Keeping Giardiasis at Bay
  • Never drink directly from a lake or stream; boil the water for one minute. (This will kill other lurking pathogens, too.) Three minutes is recommended at high altitudes.
  • Add iodine tablets to purify lake or stream water. (They're sold in most sportinggoods stores.) After a half-hour, the water is safe to drink though rather unpalatable. (If the temperature of the stream water is below 75 degrees Fahrenheit, you may need to let the iodine act overnight.)
  • To spare your taste buds and conserve your cooking fuel, buy a purification filter. Basically, this is a hand pump with a filter to strain out the nasty bits. The critical element is size: the giardia parasite is very small, only five or six microns, so your filter's pores must be no wider than one micron.
  • Guarding Against Rabies
  • Never touch wild animals: any contact with a rabid animal, dead or alive, should be considered potential exposure.
  • If you see an animal acting erratically—a raccoon wandering around in broad daylight, for example—notify an animalcontrol agency immediately.
  • If you're bitten, contact your doctor or local health department right away.
  • Avoiding Lyme Disease
  • Symptoms In its initial stages Lyme disease feels much like your basic flu: chills and fever, headache, muscle and joint pain, swollen lymph nodes. Later stages can include swollen joints, meningitis, and partial facial paralysis.
  • Protection When outdoors, tuck pant legs into socks or boots and shirt into pants (sounds dumb, but it works); wear a hat and long sleeves for added coverage. Use an insect repellent containing deet, or treat your clothes with permethrin (an insecticide), which kills ticks on contact. When hiking, walk in the center of the trail, away from overhanging grass and brush.
  • Search and Seizure There are a lot of fat, ugly ticks in the world, but the ones you have to watch out for are extremely small—the size of the period at the end of this sentence—and they like to take refuge in warm, hidden spots like your armpits, groin, and scalp. If you find one, remove it slowly with tweezers; don't squash it. Although a tick usually will fall off after four or five days, it often leaves a red, bull's-eye-shaped spot (or a series of them), which can show up three days to a month after infection. Not always, though. If you have what feels like a persistent flu, and you live in or have traveled to an area where ticks are abundant, it's worth seeing a doctor. A round of antibiotics in the early stages of Lyme disease should take care of any infection.