The invention relates to uses of nitric oxide gas to inactivate any whole, part or subunit of a microbe, such as a virus. Such an inactivated or attenuated virus after treatment with NO gas may be used in formulations for a vaccine. Nitric oxide may be administered directly to create a vaccine through in vitro and/or in situ or by direct administration in vivo. Also provided are methods for treating patients with viral infections through the inhalation of nitric oxide gas.