True about influenza ?

Ans. is 'b' i.e., Most infections are subclinical Influenza Influenza virus a RNA virus, belongs to ohomyxovirus. There are three viral subtypes : i) Type A (causes all pandemics and most epidemics); type B; and type C (not circulating currently). Currently the influenza viruses circulating in the world are : 111 N, of type A (causes swine flu); H2 N2 of type A; H3 N2 of type A ; H5 NI of type A (causes birdflu or an influenza); H7 N9 of type A (caused epidemic of an influenza in China in 2013); and type B. Influenza shows cyclic trend with epidemic occuring every 2-3 years in case of influenza - A and every 4-7 years in case of influenza-B. Pandemics are caused by only influenza - A every 10-15 years. Influenza affects all ages and both sexes. Source of infection of influenza is a clinical case or subclinical case. Major reservoir of influenza virus exists in animal and birds. Incubation period is 18-72 hours. Most of the infections are subclinical. Clinical cases present with cough, fever, myalgia and headache.