Coronaviruses (the cause of the common cold as well as SARS), parainfluenza viruses, and many other viruses all cause symptoms akin to influenza, and all are often confused with it. As a result, sometimes people designate mild respiratory infections as “flu” and dismiss them. But influenza is not simply a bad cold. It is a quite specific disease, with a distinct set of symptoms and epidemiological behavior. In humans the virus directly attacks only the respiratory system, and it becomes increasingly dangerous as it penetrates deeper into the lungs. Indirectly it affects many parts of the body,
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