The influenza virus is a contagious virus that primarily affects the respiratory system, causing the illness known as influenza, or the flu. It spreads easily through respiratory droplets when an infected person coughs, sneezes, or talks. Symptoms often include fever, cough, sore throat, body aches, and fatigue. There are several strains of the influenza virus, categorized mainly into types A, B, and C. Type A is responsible for most seasonal flu epidemics and can infect both humans and animals. Vaccines are available to help prevent infection and reduce the severity of symptoms if contracted.