rickettsial diseases, ringworm, and feline respiratory disease complex (a group of respiratory illnesses including feline herpesvirus type 1, feline calicivirus, Chlamydophila felis, and Mycoplasma haemofelis). Feline leukemia virus and feline immunodeficiency virus belong to the Retroviridae family, and both cause immunosuppression in cats, which can increase their susceptibility to other infections. Research has shown that the prevalence of these viruses among feral cat populations is low and is similar to prevalence rates for owned cats in the United States. Researchers studying 553 feral cats in North Florida in the United States tested them for a number of infections that could be detrimental to feline or human health. The study found the most prevalent infection to be Bartonella henselae, the cause of cat-scratch disease in humans, with 33.6% of the cats testing positive. Feline coronavirus was the next most common infection, found in 18.3% of the cats, although they noted that the antibody levels were low in most of the cats who tested positive, and concluded that the cats they tested did not appear to be a greater risk for shedding the virus than pet cats. Researchers studying 96 feral cats on Prince Edward Island in Canada found that feline roundworm was the most common infection in cats in that colony, afflicting 34% of cats. This was followed by Toxoplasma gondii, which was detected in 29.8% of cats, although only one cat of the 78 for whom fecal samples were available was shedding T. gondii oocysts. They did note that most fecal samples collected indicated the presence of one intestinal parasite, with some samples indicating the presence of multiple parasites. Transmission to humans The Center for Disease Control and Prevention has warned about the rabies risk associated with feral cats. With 16% of people infected with rabies from exposure to rabid cats, cats have been the primary animals responsible for transmission of the virus to humans in the United States since the efforts to control rabies in dogs in the 1970s. In 2010, there were 303 rabid cats reported within the United States. Although some colony management programs involve administering rabies vaccines, the need to revaccinate every few years makes this challenging to maintain. Furthermore, lack of documentation can mean that contact with vaccinated feral cats may still require post-exposure treatment. The study of feral cats on Prince Edward Island warned of "considerable zoonotic risk" for transmission of intestinal parasites. Although