Clinical presentations of human Babesia microti infection appear to vary among patients. The severity of the disease can be affected by several risk-increasing factors. Splenectomy appears to correlate with life-threatening and fatal cases. Simultaneous occurrence of Lyme disease and babesiosis in humans has been commonly observed in areas endemic for both infections. In addition, patients with HIV infection and AIDS have presented severe manifestations of babesiosis.
Age appears to be another important risk factor.
The intensity of clinical babesiosis caused by Babesia microti has been greater in adults over 40 years of age than in younger adults or children, although debilitating illness appears to occur among all age groups. Sufficient transmission of Babesia microti through the bite of an infective tick requires approximately 36-48 hours of attachment. Both the nymphal and adult stages of Ixodes scapularis are proven competent vectors of the infection, however, the role of adults in transmission of the infection to humans appears to be limited. On one hand, it is easier for people to notice the attachment of an adult tick, thus the tick is more likely removed before transmission occurs. On the other hand, people tend to have less outdoor activities during the cold months in early spring and late fall when adult ticks are more active.