In epidemiology a vector is an agent that carries and transmits an infectious pathogen into another living organism. Arthropods form a major group of pathogen vectors. Mosquitoes, flies, lice, fleas, ticks and mites transmit a huge number of pathogens. Lyme disease is an important vector borne disease with ixodes ticks as the vector.
The reservoir for an infectious pathogen is the habitat where the pathogen initially lives and multiplies. The reservoir is necessary for the pathogen either to survive, or multiple into sufficient amounts to be transmitted to a susceptible host.
Mice and rodents are the major reservoirs for Lyme disease. The Lyme spirochete lives and multiplies in these animal until it is picked up by a tick. Those ticks can then become infected and transmit the disease to other hosts such as humans.