Borrelia burgdorferi which can travel through the bloodstream and cause problems in the major organs of the body. Untreated. it can become a chronic disease; it may take months or even years for arthritis and other serious complications to occur.
The disease is transmitted by the bite of a tick that is infected with bacteria. The deer or black-legged tick (Ixodes scapularis) is generally responsible for transmission of the disease in the east and upper midwest; the western blacklegged tick (Ixodes pacificus) in Pacific coastal states. Research continues on other possible vectors but one must remember that even though other species of ticks and some insects may carry the spirochete, they must be able to transmit the disease to you to be considered a true vector.
The deer tick, which causes most reported cases, has a two year life cycle which can have a direct bearing on your chances of becoming infected. Eggs are laid in the spring and hatch into larvae which feed on mice and other small mammals. If the animals the larvae feed on are infected, the nymphs (which appear the next spring) will be infected and can infect other mammals, including humans.
Most cases of Lyme disease are caused by the nymph. Because of its small (pin head) size it is difficult to see or feel, and it is most active from May through July when outdoor activities are at their peak. The nymph molts during late summer into an adult, and it too can infect its new host. Because adults are active primarily in the late fall and early spring when fewer people are outdoors. and because they are easier to detect and remove due to their size, their danger in transmitting infection to humans is much less than with the nymph. However, fall hikers, hunters and those engaging in other outdoor activities at that time should be vigilant. In the northeast and upper mid-west the adults may be active any time the temperature is over 35 to 40 F., even with snow on the ground.