Virus attacks mucous membranes of upper respiratory tract, sometimes lungs. Most serious problems due to bacteria that invade while defences are weak. Death may occur in infants, elderly. Electron micrograph of papilloma virus. No proofreading to correct errors, so much higher mutation rates than DNA viruses.
Antigenic shift occurs every few years.
Viruses infect pigs and ducks as well as humans. Especially common in China. New virus strains evolve, human hosts no longer protected. Vaccines not good for more than a few years because of new strains. Has caused over 2 million deaths worldwide, number of infected individuals is growing, especially in Africa and Southeast Asia.