Ticks are not insects but are arachnids and are thus related to spiders and mites. They have 8 legs, except when they first hatch from eggs (larva), when they have 6 legs.
Blacklegged ticks, commonly called deer ticks (Ixodes scapularis), are generally born uninfected with Lyme disease, i.e., they do not contain the Lyme bacteria, Borrelia burgdorferi. Research has shown a small percent may be born infected with the bacteria.
Deer ticks feed once in each stage, larva, nymph, adult. Each time they feed, they have an opportunity to pick up Lyme bacteria from the animal they feed upon, and then next time they feed, they can pass it to whatever animal they feed upon. For example, in the Northeast, the larva is born and may feed upon the white footed mouse, vole or chipmunk, thus becoming infected with Lyme bacteria. Next time, the tick feeds as a nymph (poppy seed size tick), and it can infect whatever it feeds upon, person, dog, cat, other wild animal.
The adult has a higher infection rate than the nymph (except on the west coast), but the nymph usually produces more disease because it is smaller, poppy seed size, and harder to find on the body.
To feed, the deer tick does "questing," waiting on tall grass or on low branches or shrubs until an animal brushes by, and it can then get onto the animal. It crawls around awhile until it finds the place on the animal's body it wants to bite, which in humans is often hairy areas or folds and creases, although it will bite a person anywhere on the body.
The deer tick does have a two year life cycle and does survive the winter. Although most generally found active from March through November, the deer tick can now be found all year round when temperatures rise above 35 degrees or so.
The deer tick can carry other disease organisms besides Lyme bacteria. Babesia, bartonella, anaplasma, tularemia, and Powassan virus are some examples. One deer tick bite can infect you with one or more of these disease organisms. The deer tick can also carry a toxin which causes tick paralysis, which can only be stopped by removal of the tick.
In California and the far west, the Western blacklegged tick (Ixodes pacificus) transmits Lyme disease and other tick-borne diseases.
Other types of ticks have different life cycles and carry different diseases. Some examples important in human disease are the lone star tick (Amblyomma americanum) which transmits a Lyme-like disease called STARI; the American dog tick (Dermacentor variabilis); and the wood tick (Dermacentor andersoni).