Until 1960, dogs were the most important animal host of rabies in the United States. After the institution of mandatory rabies vaccinations for dogs, wildlife species became the most frequently infected animals with rabies. This has also been true in Michigan.

In the U.S., rabies in terrestrial mammals can be linked to distinct virus strains. Each strain is maintained primarily by intraspecies (within species) transmission although spillover infection of other species may occur. Nationwide there is one strain in raccoons that has been identified in 19 states and the District of Columbia; 3 skunk-strains; 4 fox-strains and 1 coyote-strain in Texas. Bat rabies has had distinct strains identified but geographic boundaries cannot be defined.

In Michigan, bat rabies is the prevalent strain identified.

Rabies is usually transmitted by the bite of an infected animal. Transmission has occurred by non-bite routes however in 3 instances. The first involved the presumed inhalation of virus particles in a bat cave, a second involved laboratory workers creating an aerosol and inhaling virus particles while using a power saw to cut the tops off skulls of rabies suspects, and thirdly, cornea transplants where the cornea donor had rabies. These routes of infection are not considered of general importance.