Ticks carry bacteria and other agents that cause disease. Ticks may be the primary (necessary) hosts of infective organisms which may cause illness in higher animals. Ticks are beginning to be suspected in many diseases where the pathways of transmittal seem fuzzy.
The infective agent is transmitted from tick to tick in salivary secretions. The transmission cycle of the disease in the secondary host, say a cat or human, begins with a competent (infected) tick taking a blood meal from an uninfected vertebrate, passing on the disease organism into the blood of the host. Very often the tick is unaffected by its role as a primary host.
Lyme disease, human granulocytic ehrlichiosis (HGE) and babesiosis are contracted by humans who have been bitten by ticks carrying a bacterium also carried by the white-footed mouse. The 3 diseases respond to different drugs: Lyme disease to antibiotics, HGE to only certain antibiotics, and babesiosis to anti-malarial drugs, such as quinine and one antibiotic only. The drugs induce partial recovery in a first step of stopping bacterial growth, but the patient's immune system must kill the them. Patients with weakened immune systems recover more slowly and may need more drugs.