The fisher is a rare and elusive forest carnivore that inhabits northern mature forests of North America. Unfortunately, fishers in the Pacific states have disappeared from nearly 80% of their historical range. In California, they are now restricted to a small, isolated population in the Sierra Nevada and a larger population in northwestern California. Declines in fisher populations have been linked to over-trapping for fur and loss and fragmentation of their forested habitats by logging and fires. Because of this, fishers are currently being considered for listing on the United States and California Endangered Species Acts. Disease and predation now appear to be the main causes of mortality in all California fisher populations currently under study. Fishers are highly susceptible to many diseases that can affect the health and persistence of carnivore populations. In fact, the southern Sierra Nevada fisher population recently experienced an outbreak of canine distemper virus which killed several fishers in less than a month. To determine how much disease affects California’s fisher populations and how fisher health relates to population persistence, we are examining them for detrimental diseases that could possibly threaten their populations. We work closely with several fisher field research projects in both California populations, as well as a small fisher population in Oregon, to collect and analyze biological samples that will tell us about individual fisher exposure and infection with several carnivore-specific and zoonotic pathogens. We can then use these data to assist in developing conservation plans for fishers throughout the west.