What do we do when we have cold?

Not much. : Just rest, fluids, tylenol (acetaminophen) or Ibuprofen and other symptomatic treatments. Most importantly time.

Treat symptoms: By cold i suppose you mean a viral like upper respiratory illness. These are viral in nature and will get better without treatment. However, many people have symptoms such as sore throat, nasal stuffiness, cough and headache which are best relieved with over the counter medications.