Influenza
Human influenza can pass from one person to another very easily. It is most commonly spread by the microscopic droplets of mucus and fluid that are sent into the air when the sick person coughs or sneezes. Symptoms include cough, sore throat, muscle aches and pains, fever, headache, and rarely vomiting and diarrhea.  Influenza can also lead to other diseases, such as pneumonia. This makes it especially dangerous to young children and elderly people.