What causes colds and the flu?

Answer 1

Both are caused by viruses - the common cold family of viruses (rhinoviruses and coranaviruses), and the influenza family of viruses (Orthomyxoviridae).

Both viruses are primarily via airborne droplet, but transmission via infected surfaces is also possible (but much less likely). They tend to enter through compromised mucus membrane tissue, as is common in dried-out nasal passages, or through inhalation. Virtually all infection is through the eyes, nose, or mouth. Thus, touching an infected surface then rubbing the eyes or nose is an excellent method contracting them, as is being sneezed or coughed upon by an infected individual.

Answer 2

The "Common Cold" with symptoms in the upper respiratory tract like sneezing, scratchy throat, and runny nose is usually caused by the rhinovirus (up to 40% of colds), or Coronaviruses (about 20%), but there are more than 200 viruses that can cause the common cold.

The seasonal flu, also called "regular flu", is caused mostly by Type A and Type B Influenza viruses.

Although the "stomach flu" is a common term, it is not a real diagnosis. The proper medical term for stomach flu is gastroenteritis (an intestinal disease, sometimes viral but also bacterial), it is often mistaken for influenza because true influenza can sometimes include vomiting and diarrhea.