The horses come down with strangles. The characters call it a virus when in fact it's a bacteria.
Strangles is not a virus. It is a highly contagious disease of the equine upper respiratory tract caused by the bacterium Streptococcus. The bacteria cross mucous membranes in the nose and mouth to infect lymph nodes where they cause abscesses that can eventually rupture.
How did Tim's old rodeo prize saddle survive the barn fire.