Equine Influenza (EIV) is a highly contagious respiratory disease in the horses. In the USA, the numbers of horses that have been diagnosed with EIV has increased in the past 2 years. Originally, EIV was considered a disease limited or found commonly in 2-3 year old racehorses, however, these historic population limitations have changed. Though immune-comprimised populations are more susceptible to flu, more mature and previously vaccinated horses are now susceptible to EIV. This disease is commonly associated with horses that are in co-mingling scenarios especially in the winter months. Since 2013, the UC Davis veterinary school's diagnostic lab has identified 16 EIV outbreaks in ten US states including Oregon, California, Idaho, Nevada, Montana, Utah, Arizona, Texas, Oklahoma, and Florida. Horses that are housed together, horses with a possible compromised immune system such as horses in transport or competition stress may be more prone to contracting this disease. Vaccinating against EIV is highly recommended since this disease can be highly fatal.