The pathogen that causes the flu is a virus called influenza, according to the University of Maryland Medical Center. Every year, millions of people around the world come down with the flu.
A virus is simply a mass of genes surrounded by a protein coat, states the University of Maryland Medical Center. Of the two types of influenza virus strains, the influenza A strain can infect both humans and animals, whereas the influenza B strain can only affect humans. Influenza A causes more flu cases and is more serious. Some symptoms of the flu are a high fever, headache, body aches, exhaustion, coughing, sneezing, a stuffy nose and a sore throat. Some forms of flu, such as the avian flu, can be transmitted from animals to humans.