Two of the three families of ticks are capable of transmitting diseases to humans: Ixodidae, or hard ticks, and Argasidae, or soft ticks. The photomicrograph presented here is a close-up view of the piercing mouthparts from Dermacentor variabilis, commonly known as the American dog tick.

This pest belongs to the hard tick family, which also includes the deer tick (Ioxdes scapularis) and the lone star tick (Amblyomma americanum). These ticks have four life stages: egg, larva, nymph and adult. Each stage requires a blood meal for morphogenesis. While feeding, a tick may stay attached to its host for hours or even days.

In the United States, ticks are the number one vector of infectious disease and are second only to mosquitoes worldwide. Hard ticks are responsible for transmitting nearly all of the major tick-borne diseases in North America. Ticks can maintain pathogens in various ways. Some microorganisms infect the eggs, which hatch into larvae, then continue through the various life stages. Other organisms are acquired by feeding on an infected host and can be transmitted to an uninfected host.

The American dog tick is one of the ticks that harbors the organism causing the Rocky Mountain Spotted Fever, the most common tick-borne disease in the United States. The illness was first spotted in Montana and Idaho, although it is now prevalent in southeast. Symptoms include sudden high fevers, severe headaches, fatigue, muscle aches, nausea and sometimes rashes. Usually symptoms become apparent three to 12 days after the victim has been bitten. As the organisms spread through circulation they cause bleeding of microscopic vessels that consequently can cause the vascular system to collapse, leading to death. If diagnosed early, antibiotics can cure the illness.