Anaplasma phagocytophilum is a type of bacteria that can infect white blood cells in humans and animals. It is primarily transmitted through the bite of infected ticks, particularly the black-legged tick and western black-legged tick. This bacterium is responsible for a disease known as human granulocytic anaplasmosis (HGA), which can cause symptoms like fever, headache, and muscle aches. Infection with Anaplasma phagocytophilum can lead to complications if not treated promptly. Diagnosis is typically made through blood tests that identify the bacteria or antibodies produced in response to the infection. Treatment usually involves antibiotics, which are effective in clearing the infection when administered early.