A、The eight-month-old baby. B、Teenagers. C、Mid-agers. D、People in their 50s. A

Influenza is a common infection of the nose and throat, and sometimes the lungs. The flu causes muscle pain, sudden high body temperature, breathing problems and weakness. It is especially dangerous to the very. young, the very old and those with a weak defense system against diseases.
    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. It spread in Asia in eighteen-twenty-nine, then again in eighteen-thirty-six. It also traveled to Indonesia, Russia and the United States.
    In eighteen-eighty-me, the flu began in Central Asia, spread north into Russia, east to China and west to Europe. Later, it affected people in North America and Africa Experts say two-hundred-filly-thousand people died in Europe in that flu. Worldwide, the number was at least one-million.
    But the deadliest outbreak of influenza on record 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. Even young, healthy people became sick and died in just a few days.
    Eighty years ago, the flu virus took months to spread around the world. Today, airplane travel means a virus can spread around the world within days. Experts say another virus like the one that appeared in nineteen-eighteen could be as dangerous as any disease ever known.