Ehrlichiosis is caused by ehrlichia bacteria and is transmitted primarily by the Lone Star tick.
Ticks feed on blood, latching onto a host and feeding until they're swollen to many times their normal size. During feeding, ticks that carry disease-producing bacteria can transmit the bacteria to a healthy host. Or they may pick up bacteria themselves if the host, such as a white-tailed deer or a coyote, is infected.
Usually, to get ehrlichiosis, you must be bitten by an infected tick. The bacteria enter your skin through the bite and eventually make their way into your bloodstream.
Before bacteria can be transmitted, a tick must be attached and feeding for at least 24 hours. An attached tick with a swollen appearance may have been feeding long enough to have transmitted bacteria. Removing ticks as soon as possible may prevent infection.
It's also possible that ehrlichiosis may be transmitted through blood transfusions, from mother to fetus, and through direct contact with an infected, slaughtered animal.