Babesiosis, a potentially life-threatening tick-borne disease, is a rare but grave complication that can be transmitted through blood transfusions, the U.S. Centers for Disease Control and Prevention warns.

It has infected at least 122 people since 2000, says a study by researchers at the CDC.

Babesiosis is a malaria-like illness transmitted to humans by the black-legged tick, also known as the deer tick. Babesia parasites typically invade their host as a result of a tick bite, but they can also be transmitted by blood transfusion.

The finding doesn't mean transfusions are dangerous, only that physicians need to be aware of a previously invisible illness.

In the U.S., babesiosis is caused by two species of parasite: Babesia microti and Babesia duncani.

The symptoms of babesiosis usually appear one to four weeks after infection, but can come much later. They resemble flu symptoms and often include fever, headaches and muscle aches, sweating, fatigue, nausea and vomiting and weakness.

The CDC says people at the highest risk include those without a spleen or whose spleen functions abnormally, as well as those with compromised immune systems or who have other serious health problems, such as kidney or liver disease. The disease can be life-threatening in people whose immune system has been weakened by cancer, AIDS or another medical condition.

There's no Food and Drug Administration approved test for the parasite in blood supplies, so blood banks rely on asking would-be donors about flu-like illnesses and tick bites.

The actual number of cases that occur in a given year is unknown. The disease, even in cases that cause severe symptoms, are often missed or misdiagnosed.