A cold doesn't necessarily ‘turn into' pneumonia, but can create an environment in which viruses or bacteria can attack the lungs causing pneumonia.
The most common cause of pneumonia is bacteria. In this case, though the cold virus itself does not cause the pneumonia, it could ultimately irritate the lungs, lowering tolerance making you more susceptible to infection. This is especially true in those who are very young, elderly, or have weakened immune systems.
One-third of pneumonia cases are caused by viruses, one of which is the rhinovirus, which can also cause the common cold.
The Common Cold vs. Pneumonia
When first beginning to feel the onset of a respiratory illness, you most likely will not be able to tell whether it's just a cold or something more serious like pneumonia. Many of the symptoms mimic each other such as fever, cough and chills. However, the common cold and its symptoms usually do not stick around for long and usually do not require anything more than over-the-counter medications, liquids, and bed rest.
You're Still Sick after Two Weeks
If your ‘cold' symptoms do not improve within a few weeks of over-the-counter medications, bed rest and other common cold remedies, you may have a more severe illness such as pneumonia. Since the early stages of pneumonia can mimic the common cold, without a visit to a doctor it is unlikely that you will be able to diagnose yourself as having a cold, influenza, bronchitis or pneumonia.