True about influenza -

Ans. is 'b' i.e.. Most infections are subclinical Influenzao Influenza virus a RMA virus, belongs to orthomyxovirus.o There are three viral subtypes : i) Type A (causes all pandemics and most epidemics); type B: and type C (not circulating currently).o Currently the influenza viruses circulating in the world are : Ht N, of type A (causes swine flu): H2N2 of type A; H3N2 of type A ; H5 N1 of type A (causes birdflu or avian influenza); H7 N9 of type A (caused epidemic of avian influenza in China in 2013); and type B.o 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,o Influenza affects all ages and both sexes.o Source of infection of influenza is a clinical case or subclinical case,o Major reservoir of influenza virus exists in animal and birds.o Incubation period is 18-72 hours. Most of the infections are subclinical. Clinical cases present with cough, fever, myalgia and headache.