Measles (MeV) causes disease worldwide despite efforts towards eradication by vaccine, largely because it is spread so readily between people. Acute MeV infection causes immune amnesia, resulting in increased susceptibility to other infectious diseases. In addition, rare but severe neurological complications can develop several years after measles due to persistent MeV infection of the central nervous system. People with impaired cellular immunity are at increased risk of developing severe measles, but often cannot be vaccinated since the vaccine virus itself can lead to fatal illness. There is no specific therapy for acute or persistent MeV manifestations. A successful vaccination campaign could have eradicated MeV more than 20 years ago. As of today, eradication is not in sight and the resurgence of measles in the U.S. 2019 highlights the need for effective measures to prevent host-to-host transmission at the moment of the outbreak surge. We have recently described a new MeV-specific fusion inhibitor peptide that combines viral-specific targeting, self-assembling, and cell- membrane integration leading to a MeV fusion inhibitor that outperformed our previous fusion inhibitors in vitro and in vivo. This application will test whether this new inhibitor prevents inter-host transmission and therefore fill this medical demand. We propose to chemical engineer these inhibitors to optimize 1) the viral-specific targeting, 2) the insertion on infected cells membrane, and 3) in vivo biodistribution. Our work will be tested in vitro, ex vivo, and in vivo using a natural model of morbillivirus infection (Canine Distemper Virus -CDV- in Ferrets). 1. To use protein engineering to optimize the self-assembling properties and antiviral potency of HRC-peptide fusion inhibitors. 2. To evaluate the protection afforded by HRC peptide fusion inhibitors against CDV infection in vivo and provide proof of concept for pre-clinical development.