Flu pandemic. Conceptual animation of a virtual Earth globe with an influenza virus particle growing inside it. The virus particle eventually engulfs the globe. Influenza is in the orthomyxovirus group of viruses. It infects the respiratory tract and causes fever, aching muscles, a sore throat and feelings of fatigue. New strains of the virus develop rapidly, and can cause epidemics. Global epidemics are known as pandemics and can affect millions.