By a tick bite. Rocky mountain spotted fever (rmsf) is a rickettsia (type of bacteria) infection transmitted by a tick bite (feeding > several hrs). Symptoms start in 1 week (2-14d range). The germ attacks the cells lining the small blood vessels in the body, causing multi-organ damage and maybe death. Very rarely, rmsf can be transmitted by a blood transfusion or accidental exposure in an infectious disease lab.
Tick Bites. The bacteria is rickettsia rickettsii and carried by ticks which spread it when they bite. Crushing a tick attached to a pet can also transit the bacteria into you. In the west the ticks are carried by wood ticks and in the east they are carried by dog ticks. Only 1/1000 ticks carry the bacteria and usually the bacteria seems to transmit only when the tick been attached to you for over 20 hours.