Col. David Avila, Nevada Test and Training Range commander receives a flu vaccine from Staff Sgt. Mark Butanis, 99th Aerospace Medical Squadron medical technician, on Nellis Air Force Base, Nev., Sept. 29. The flu, also known as influenza, is a highly contagious virus that is spread every year, usually between October and May. The flu is passed along mainly by coughing, sneezing and close contact. Flu vaccinations are mandatory for all active duty members and general schedule civilian employees, but are highly recommended for everyone.