They can wait for months, clinging to the edge of a blade of grass or a bush, for the whiff of an animal's breath or vibration telling them a host approaches.
They are ticks—and when they attach to your skin and feed on your blood over many days, they can transmit diseases. Often hard to diagnose and tricky to treat, tick-borne illnesses—led by Lyme disease—can cause symptoms ranging from headache and muscle aches, to serious and long-term complications that affect the brain, joints, heart, nerves and muscles. Preventing bites to head off illness is particularly important, experts say, because the complex interaction between ticks, their hosts, bacteria and habitats isn't completely understood.
Warmer temperatures are leading some experts to warn that tick activity is starting earlier than usual this year, putting more people at risk.
"This is going to be a horrific season, especially for Lyme," says Leo J. Shea III, a clinical assistant professor at the Rusk Institute of Rehabilitation Medicine, part of New York University Langone Medical Center. He is also president of the International Lyme and Associated Diseases Society.
Lyme may be identified after a tick bite, for example, by an expanding rash that looks like a bull's-eye. But that doesn't always happen, and even after a tick bite, antibodies against Lyme may not show up for weeks, so early blood tests can turn up false negatives. Symptoms such as fatigue, chills, fever, headache and swollen lymph nodes may be misdiagnosed. Some infections can go undetected for months or even years. When caught early, tick-borne diseases can be treated successfully with two weeks of antibiotics, but doctors and researchers still argue about whether a chronic form of Lyme exists, and whether it should be treated with longer courses of the drugs.
Between 1992 and 2010, reported cases of Lyme doubled, to nearly 23,000, and there were another 7,600 probable cases in 2010, according to the Centers for Disease Control and Prevention. But CDC officials say the true incidence of Lyme may be three times higher. Other infections, including babesiosis, Rocky Mountain spotted fever, and anaplasmosis are steadily increasing, too. While not all ticks carry disease, some may spread two or three types of infections in a single bite.
Researchers say the primary reasons for the global rise of tick-borne illness include the movement of people into areas where animal hosts and tick populations are abundant, and growth in the population of animals that carry ticks, including deer, squirrels and mice.
"We haven't even begun to scratch the surface of the type of pathogens ticks can be harboring and transmitting," says Kristy K. Bradley, state epidemiologist and public health veterinarian for the Oklahoma State Department of Health.
Animals "are a traveling tick parade," Dr. Bradley adds, with pet dogs "bringing them into the home and onto furniture and carpets."
Regularly checking the body for ticks can reduce exposure, because removing them quickly can prevent transmission of disease, says Kirby C. Stafford III, chief entomologist at the Connecticut Agricultural Experiment Station, or CAES, in New Haven.
Showering or bathing quickly after being outdoors can also help wash off crawling ticks or make it easier to find them. What won't work: simply jumping in the pool or lake, because ticks can hide in bathing suits and don't quickly drown in water. There are tick-repellent sprays for clothes, but it is wise to immediately launder and dry garments at high temperatures after hiking or golfing in areas where ticks are present.
The CDC is conducting the first study of its kind to determine whether spraying the yard for ticks can not only kill pests, but also reduce human disease. Participating households agreed to be randomly assigned a single spray with a common pesticide, bifenthrin, or one that contained water, without knowing which they would receive.
Paul Mead, chief of epidemiology and surveillance activity at CDC's bacterial-illness branch, says preliminary results from about 1,500 households indicate that a spray reduced the tick population by 60%.
"But there was far less of a reduction in tick encounters and illness," indicating that even a sharp drop in tick populations leaves infected ones behind. "We may have to completely wipe out ticks to get an effect on human illness," he says. The CDC is enrolling households for a second arm of the study and expects final results late in the fall. Organic repellents such as Alaska cedar are also being tested in other studies.
Sometimes fire is the only solution: Wildlife biologist Scott C. Williams roams Connecticut's woods armed with a propane torch to incinerate clumps of Japanese barberry, an invasive plant species that chokes off native vegetation and provides a favorite habitat for ticks.
The CAES program to control the red-berried shrub—once cultivated as decorative—is part of the growing, multifaceted effort around the country to prevent the spread of infections like Lyme, which Dr. Williams has been treated for twice since beginning the project in 2007.
Dr. Bradley's home state of Oklahoma is one of several working with the One Health Initiative, a global program to improve communication between physicians and veterinarians to prevent the spread of infectious disease from animals to people, such as recommending tick collars, sprays or topical treatments with pesticides for dogs.
One problem, says Laura Kahn, a founder of One Health, is that "vets don't like to advise people on human health and physicians don't typically think about these things, so it falls through the cracks." About 75% of new diseases that have emerged globally in the last 30 years are spread from animals to people, many of them through ticks, says Dr. Kahn, who is also a science-and-global-security researcher at Princeton University.
Jason Lipsett, 21 years old, was diagnosed with Lyme in November, after suffering for three years with symptoms including problems with his jaw, recurring sinus infections, migraines and trouble sleeping. He had to give up playing tennis and take a medical leave from Bentley University in Waltham Mass., where he was a senior. He doesn't remember being bitten by a tick but had been camping in the woods in New Hampshire and often spent time outdoors during the summers at a family home in Cape Cod.
Doctors told him he might have chronic fatigue syndrome or fibromyalgia. Depressed about his health, he began seeing a therapist who knew about the symptoms of Lyme and referred him to another physician. That doctor determined he had Lyme—and babesiosis, caused by a parasite that destroys red blood cells.
Mr. Lipsett has been on an antibiotic regimen for four months. He says he has felt better each month and that he is prepared to stay on the drugs until he and his doctor are confident the disease is under control. He is making up courses and hopes to graduate next year. He plans to participate in a 5K run on April 29 to raise money for Time for Lyme, a Stamford, Conn. nonprofit that supports research into Lyme and other tick-borne illnesses.
"I may not be able to run, but I'm going to try to walk it," he says.
Write to Laura Landro at firstname.lastname@example.org
A version of this article appeared Mar. 27, 2012, on page D1 in some U.S. editions of The Wall Street Journal, with the headline: This Season's Ticking Bomb.