Chest infection (respiratory tract infection) refers to any infection within the large airways and the lung tissue. It can be caused by a variety of germs, and while some chest infections are mild and clear up on their own, others can be severe and life threatening. A cold is an inflammation of the tissues in the nose and upper airways often due to a viral infection. It often settles without any treatment. Flu (influenza) is a highly infectious condition often caused by viruses. While it involves the airways, it can quickly involve other parts of the body and can be severe in the very old or young.