Ehrlichia chaffeensis is a type of bacteria that causes human monocytic ehrlichiosis, a disease transmitted primarily by the bite of infected ticks. This bacterium infects white blood cells, leading to symptoms such as fever, fatigue, and muscle aches. The disease is most commonly reported in the southeastern and south-central regions of the United States. Diagnosis is typically made through blood tests, and treatment usually involves antibiotics like doxycycline. Early detection and treatment are crucial to prevent severe complications.