Feline infectious peritonitis (FIP), caused by the mutant feline coronavirus known as FIPV, is a relatively common and generally lethal disease of domestic cats. Recent clinical trials carried out at the University of California, Davis have identified several antiviral compounds with the ability to cure subsets of cats with this deadly disease. However, animals with brain and/or eye involvement have proven difficult to treat using a single antiviral compound alone (monotherapy). We propose that a combined antiviral approach where two or more antiviral compounds are concurrently administered will result in a higher cure rate compared to monotherapy. This combined antiviral approach is modeled after successful combined antiviral regimens used to treat human viral diseases including HIV and viral hepatitis. Alongside this clinical trial, enrolled client-owned cats will help to answer important questions on how the FIP virus infects, spreads throughout the cat and causes persistent disease (pathogenesis). A more nuanced understanding of FIP pathogenesis may provide critical insights into prevention and/or treatment of this devastating feline disease.