The ability of the virus to genetically change results in a selective advantage over the immune surveillance of the host. We now appreciate that small mutations in flu virus genes result in new antigens that are termed antigenic drifts. Major mutational changes, or antigenic shifts, occur only in Type A viruses and can result in pandemics. The hemagglutinin gene product in the 1968 virus strain was so different from the protein found in the 1957 strain that minimal immunity existed in the population and the pandemic was very severe. The Russian flu pandemic of 1977 affected primarily people younger than 25 years of age, indicating that people who had been exposed to the flu strain before 1950 had developed and maintained immunity.
Infection is considered contagious for another three to four days after symptoms appear.
Fever starts declining typically on the second or third day of illness, replaced by nasal congestion and sore throat. Diagnosis is made usually by symptoms and an existing community epidemic. Treatment consists of rest, fluids and aspirin or acetaminophen, but not antibiotics, which are not effective in treating viral diseases. Type A infections in adults. When taken within 48 hours after onset of illness, it reduces duration of fever and other symptoms.