Flu is an infection of the nose, throat and lungs, which are part of the respiratory system. Influenza is commonly called the flu, but it’s not the same as stomach “flu” viruses that cause diarrhea and vomiting.
The symptoms of viral pneumonia usually develop over a period of several days. Early symptoms are similar to influenza symptoms: fever, a dry cough, headache, muscle pain, and weakness. Within a day or two, the symptoms typically get worse, with increasing cough, shortness of breath and muscle pain.