Babesiosis is caused by microscopic parasites that infect red blood cells and are spread by certain ticks. In the United States, tickborne transmission is most common in particular regions and seasons: it mainly occurs in parts of the Northeast and upper Midwest and usually peaks during the warm months.

Although many people who are infected with Babesia do not have symptoms, for those who do effective treatment is available. Babesiosis is preventable, if simple steps are taken to reduce exposure to ticks.

Ticks in the nymphal stage are usually the size of a poppy seed and can transmit Babesia microti if infected.