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A parasitic infection normally transmitted by deer ticks has made its way into the U.S. blood supply, researchers from the Centers for Disease Control and Prevention reported on Tuesday.

The infection is called babesiosis and it’s increasingly spreading through blood transfusions. The first case of babesiosis transmitted through transfusion occurred in 1979, according to the CDC. Since then, another 161 cases have been documented — 77% of them in the last decade.

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“It’s uncommon, but important,” said the CDC’s Dr. Barbara Herwaldt, the lead author of the report, noting that the overall risk of getting babesiosis through blood transfusion remains small. “People should not be afraid of transfusions.”

All but three of the cases recorded by the CDC were caused by the Babesia microti parasite. Typically, people who are infected with B. microti get it from deer ticks, which are present mostly in seven states in the Northeast and upper Midwest. About 87% of the transfusion-related transmissions recorded in the CDC report also occurred in these areas, but because blood products are shipped nationwide, the cases haven’t been limited by geography. (The other three transfusion-related cases were caused by B. duncani, a species of Babesia found in western states.) Cases occurred in all four seasons even though tick bites are seasonal.

Although a history of babesiosis prohibits blood donation, many potential donors don’t know they’ve been infected because the symptoms of babesiosis can be mild or nonexistent. And while doctors can run tests that detect the parasite or antibodies to it in the blood if an infection is suspected, there are currently no approved screens being used routinely for potential blood donors.

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The authors of the study say better prevention strategies, including the development of a screening test, are needed. In the meantime, they urge better awareness of babesiosis symptoms, which can include fever, headache, muscle aches, sweating, fatigue, nausea and weakness, and usually appear one to four weeks after infection. The disease can also cause unexplained anemia, since the parasite invades and destroys red blood cells.

Often, doctors and patients mistake babesiosis for the flu. Even when a babesiosis diagnosis is considered, it is often erroneously diagnosed as malaria.

The disease is treatable with antibiotics, but it can become severe or even deadly in certain vulnerable populations, including the elderly, those with weakened immune systems and patients who don’t have a spleen.

It’s not clear whether the rise in babesiosis infections recorded by the CDC means that the risk is increasing in some areas or that growing awareness has led to an uptick in diagnoses, but, as the researchers note in the study, the transfusion-related cases they “enumerated undoubtedly represent a fraction of those that occurred.”

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To prevent becoming infected with babesiosis — and keeping it out of the blood supply to begin with — people should avoid tick bites by wearing protective clothing and using insect repellent.

The CDC study was published in the Annals of Internal Medicine.

Sora Song is the editor of TIME Healthland. Find her on Twitter at @sora_song. You can also continue the discussion on Healthland’s Facebook page and on Twitter at @TIMEHealthland.