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Despite its many delights, summer also brings its fair share of pestilence. One, called babesiosis, has only recently been widely recognized as a potentially serious outdoor hazard. According to a very detailed study conducted on Block Island, R.I., it could eventually rival Lyme disease as the most common tick-borne ailment in the United States.

But with reasonable precautions, neither babesiosis nor Lyme should keep you from enjoying a romp in the grass or hike in the woods.

Personal Health Jane Brody on health and aging.

Babesiosis is caused by protozoans that invade red blood cells and can cause a malarialike illness. The disease has an interesting history, recently recounted in The New England Journal of Medicine by Dr. Peter J. Krause, a Yale researcher specializing in tick-borne diseases, and Edouard Vannier, an immunologist at Tufts Medical Center.

Babesiosis (pronounced buh-BEEZ-e-OH-sis) is named for Dr. Victor Babes, a Romanian pathologist who in 1888 identified the disease in cattle that had fever and blood-tainted urine. Until the mid-20th century, the disease was known only in wild and domestic animals, which can be infected by more than 100 different Babesia species.

The first human case was not recognized until 1957. A Croatian herdsman who had no spleen, an important immunological organ, died quickly of the infection, which he most likely acquired from the animals he tended. Twelve years later, the first case in an immunologically normal person was identified on Nantucket Island, and for years the disease was called Nantucket fever.

Unlike Lyme disease, which quickly leapfrogged across the country, babesiosis is spreading slowly through the Northeast and Upper Midwest, where it is increasingly recognized as the cause of a flulike summer ailment. It has been said that Lyme disease moves on the wings of birds, which some experts believe carry the bacteria causing the condition. Babesiosis, however, moves on the backs of mice and deer. Birds do not spread it.

But like the bacteria that cause Lyme disease, Babesia protozoans are transmitted to humans by ticks, which acquire the infection from the white-footed mouse and white-tailed deer. And, yes, the same tick — Ixodes scapularis, popularly called a deer tick — transmits both Lyme disease and babesiosis in this country.

The deer tick, which starts out the size of a poppy seed, requires a blood meal at every one of its developmental stages. With its hind legs clutching grass or a leaf, the tick sits patiently, holding its pincerlike front legs extended, ready to latch onto an unsuspecting mammal that happens by.

In spring or summer, that mammal could be you. In the fall, the adult tick feeds on white-tailed deer, which don’t get sick. This feeding allows female ticks to produce a profusion of eggs for the next generation.

Still Uncommon, but Worrisome

“Babesiosis is already a worldwide disease, though the United States has the most cases so far,” Dr. Krause said in an interview. “Its geographical distribution is growing, and we think over time it will become increasingly important relative to Lyme disease.”

In 2011, the first year of national surveillance, only 1,000 cases of babesiosis were reported. But Dr. Krause’s study on Block Island, which tracked the risk of infection among 70 percent of the people living there, revealed that babesiosis was one-third as common as Lyme among those who developed symptoms and nearly as common as Lyme in asymptomatic people.

Underreporting of babesiosis is expected to continue for a long time. People with only mild symptoms are unlikely to see a doctor, and without a telltale sign like the bull’s-eye rash of Lyme or a simple blood test for the infection, most doctors are unlikely to diagnose babesiosis correctly, Dr. Krause said.

One-quarter of infected adults and half of infected children were free of symptoms, his study found. Yet if they donated blood, they could transmit the infection to others, with potentially dire consequences. There is no widely used test to screen blood donors for infection with Babesia.

Likewise, an infected woman could transmit the protozoans to her child during pregnancy or delivery.

The risk of a severe and possibly fatal infection is highest in newborns, adults over 50 and anyone with compromised immunity, including people with cancer, H.I.V. or a transplanted organ, and those missing the spleen.

Dr. Krause explained that the spleen “helps to clear organisms in blood that shouldn’t be there.”

“It produces antibodies that attack the protozoans, which are then gobbled up by macrophages, and it acts like a sieve, screening out Babesia-infected blood cells, which are too big to get through and back into circulation,” he said.

Prevention and Treatment

As with Lyme disease, precautions to prevent the bite of a Babesia-bearing tick include staying on cleared trails to minimize contact with leaf litter, brush and tall grass; wearing socks with long pants tucked into them and long-sleeved shirts (not the most pleasant approach on a steamy summer day); and applying repellent to exposed skin and clothing. Products containing DEET can be applied directly to the skin and sprayed on clothing; those containing pyrethrins should be used only on clothing and shoes.

Daily tick checks should be as routine as brushing teeth for people in environments that could harbor ticks. Check everywhere, using mirrors if necessary, including underarms, groin, navel, back of the neck, behind knees, between toes, behind and in ears, and on the scalp.

If a tick is found, without delay use fine-pointed tweezers to grasp it close to the skin and pull upward with steady, even pressure. Do not twist or yank it. Then clean the area and your hands with rubbing alcohol or soap and water.

People who are infected can become ill one to four weeks after a tick bite. Common symptoms include fever, malaise, fatigue, chills and sweats, headache, muscle and joint pain, loss of appetite, cough and nausea. A blood test may reveal anemia.

Certain diagnosis comes from detecting the protozoan in a blood smear. Dr. Krause suggested that labs examine 300 microscopic fields before ruling out the disease.

While the infection clears in some people without treatment, most require a combination of antibiotics, usually atovaquone (Mepron) and azithromycin (Zithromax), for 7 to 10 days. Dr. Krause said even patients with mild symptoms should be treated because they may become severely ill at a later time or spread the infection to others through donated blood.