Anaplasma phagocytophilum is a type of bacteria that primarily infects white blood cells in mammals, including humans. It is 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 lead to symptoms such as fever, headache, and muscle aches. Infection with Anaplasma phagocytophilum can be diagnosed through blood tests that identify the bacteria or antibodies produced in response to the infection. Treatment typically involves antibiotics, such as doxycycline, which are effective in clearing the infection. Early diagnosis and treatment are crucial to prevent complications.