a) The two diseases caused by the virus are common cold, poliomyelitis

The common cold is caused by a virus and easily gets spread through the air, direct contact with infected person etc. Poliomyelitis is caused by polio virus and Salk’s vaccine is given to prevent this disease.

b) One important characteristic of a virus is that it can reproduce only inside a host cell.

The host cell may be a plant or animal or bacteria.