The afebrile period lasts about a week before there is a second attack of fever, which is followed by another afebrile period. Generally there are 3 - 10 such episodes, of diminishing severity. More serious illness can occur if there is extensive growth of bacteria in the spleen, liver and kidneys. The bacteria responsible for this disease are from the genus Rickettsiae carried by dog or wood ticks. The disease is transmitted from generation to generation or from adult tick to egg.
Human infection occurs in the warm months of the year as ticks become active.
Children are most commonly infected, but their disease is milder. The rickettsiae multiply in the skin at the site of the tick bite, then spread to blood and infect vascular endothelium in the lung, spleen, brain and skin. After an incubation period of about one week there is onset of fever, severe headache and myalgia, and often respiratory symptoms. A generalized maculopapular rash appears a few days later, often becoming petechial or purpuric. This may include the palms and the soles of the feet.