According to Flu, a U.S. Department of Health and Human Services website, influenza A's symptoms are similar to other strains of flu, consisting of fever, chills, coughing, sore throat, runny or congested nose, headaches and fatigue. Vomiting and diarrhea occurs primarily in children and occasionally in adults.
Influenza A can infect animals, though WebMD says this is less common. It is typically transmitted through wild birds or other people. Of the three types of flu, only types A and B can affect humans with any severity.
The influenza A virus and its variants change constantly, which is why medical professionals emphasize the need to receive the flu vaccine yearly. As an example, in 2009 when the so-called "swine flu" began to spread, the virus strain differed significantly from typical flu and caused the first flu pandemic in 45 years. While there is no vaccine for avian or swine flu, the yearly vaccine recommended by the CDC contains protections against H1N1 and H3N2 viruses.
The World Health Organization warns that Type A flu has a higher incidence of viral pneumonia and higher fatality results among healthy young adults. It occurs more frequently in the winter season. As with any type of influenza, Type A flu is transmitted through any surface that an infected person has touched.