Influenza is a common infection of the nose and throat, and sometimes the lungs. It is caused by a virus which passes from one person to another.The flu causes muscle pain, sudden high body temperature, breathing problems and weakness. It is most common in the winter months.Generally, people feel better after a week or two. But the flu can kill. Historical records have described sicknesses believed to be influenza for more than two-thousand years. The Roman historian Livy described such a disease attacking the Roman army. People in fifteenth century Italy thought the sickness was caused by the influence of the stars. So they named it "influenza." In seventeen-eighty-one, influenza went from Europe to North America to the West Indies and Latin America.In eighteen-eighty-nine, the flu began in Central Asia, spread north into Russia, east to China and west to Europe. But the deadliest outbreak of influenza on record involved a flu that first appeared in Spain. The so-called Spanish flu killed between twenty-million and fifty-million people around the world in nineteen-eighteen and nineteen-nineteen.