Influenza develops after the virus enters a person's nose or mouth. It causes muscle pain, sudden high body temperature, breathing problems and weakness ___________. But it can be very dangerous. It is especially dangerous to the very young, the very old and those with weakened defences against disease.

• A. The virus stopped spreading completely after the treatment
• B. Generally, most people feel better after a week or two
• C. Then, they changed the temperature and humidity levels
• D. But they did not really know why until recently