Human erythrocytes (red blood cells) showing infection with Babesia microti.
Note that the ring forms with a central pale area inside the red blood cell (arrows).

Babesiosis is a parasitic infection caused by protozoa of the genus Babesia and transmitted by ticks; humans are infected incidentally and initially develop a nonspecific febrile illness. Babesia organisms enter red blood cells, or erythrocytes, and resemble malarial parasites morphologically, thus posing a diagnostic problem. The disease most severely affects patients who are elderly, immunocompromised, or have undergone splenectomy. Babesiosis is usually an asymptomatic infection in healthy individuals.

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