HME is caused by Ehrlichia chaffeensis and, in the United States, generally occurs in southeastern, south-central, and mid-Atlantic states from May to July. The incidence can be as high as 414 cases per 100,000 population.

• E. chaffeensis is transmitted by the Lone Star tick (Amblyomma americanum), and white-tailed deer are the major reservoir.
• The median age of pts is 52 years; 60% of pts are male.
• E. ewingii and an E. muris-like agent cause an illness similar to, but less severe than, that due to E. chaffeensis.