Feline infectious peritonitis (FIP) remain a leading cause of death and disease in cats. Antiviral drugs targeting feline coronavirus (which is the cause of FIP) currently show great promise for treating FIP, but are not currently approved for use by veterinarians, are expensive and are likely to need to be used in combination with other drugs to prevent the problem of drug resistance. In this study, we will test the use of a common antibiotic, doxycycline, for its ability to acts as an antiviral as well as an antibiotic (anti-bacterial). We believe doxycycline is an ideal first candidate for combination therapy in cats, which will be ultimately necessary to combat FIP. Doxycycline is highly cost-effective, and licensed and approved for use in cats. This study will provide strong functional evidence to support the use of doxycycline in future clinical trials of FIP.