Some differences between strains of influenza include how easily they spread, whether they can infect animals and severity of symptoms, according to WebMD. Of the three strains of influenza virus, labeled A, B and C, only type A and B are responsible for annual epidemics.
All three types of viruses infect humans and cause symptoms that include fever, body aches, sneezing and coughing. Type A influenza is the most likely of three strains to create a widespread epidemic, WebMD reports. Type A is a flu virus that is constantly changing and can be spread to animals. Wild birds are often hosts of this virus.
Type B influenza usually creates less severe symptoms than type A, WebMD states. It only affects humans and is not a virus that causes pandemics. Sometimes variants of type B flu can become dangerous. The mildest strain of influenza is type C. This flu type has mild symptoms and doesn't reach epidemic proportions. It is also only found in humans.
Flu strains mutate from year to year, which is why adults are encouraged to receive an influenza vaccine every year, according to WebMD. To help create the most effective vaccine for each year's flu season, scientists predict the most likely variants of A and B flu strains to be the most potent based on international surveillance.