Ehrlichiosis is a bacterial infection spread by ticks. There are five distinct species of bacteria that cause the condition which act in the same manner. The bacteria directly attack white blood cells, making the condition a particular threat to the usual mechanisms used by the immune system. This can result in secondary opportunistic infections such as candidiasis. In severe cases, the bacteria's effect on the immune system can cause toxic shock, leading to death. However, this is rare and usually occurs in patients who are already immunocompromised.