Loeffler and Frosch proved that foot-and-mouth disease is caused by a filter-passing virus; this was the first recognition that a virus causes disease.
Marek's disease (fowl paralysis) later named by Peter Biggs, who determined that the disease was caused by Gallid alphaherpesvirus 2, and developed the first generation Marek's disease vaccine.
The authors were the first to isolate a pathogenic coronavirus in chickens. This was the first pathogenic coronavirus described. (Thanks to Juan Weiss for this reference and its interpretation.)
