Type A and B influenza are the most common strains and cause seasonal flu and flu pandemics, while Type C influenza is less common because it is not related to the seasonal flu. Type A can occur in humans and animals, while Type B only occurs in humans, says Healthline.
Type A influenza strains have a higher likelihood of causing symptoms in humans and are usually responsible for flu pandemics. Type B flu usually causes less severe symptoms than type A. Type C flu is also found in humans, but causes milder symptoms than Types A and B and is not known to cause pandemics, as reported by WebMD.