What’s the difference between a cold and the flu?
The essential difference between a cold and the flu is that they are caused by different viruses. In fact, there are several families of viruses that cause colds (which is why colds are more common than the flu), whereas the flu is always caused by the influenza virus.
From an experiential perspective, the differences get a bit trickier. Though symptoms vary from person-to-person and there is some symptom overlap (like getting a sore throat or a cough), most people experience different symptoms when they have the flu compared to when they have a cold. For example, characteristic symptoms of the flu– like severe body aches and headaches, as well as persistent high fever–don’t often surface with a cold.
Generally speaking, the duration of the flu is longer and its symptoms are more severe than what you’d see in a cold. Colds usually knock people out for a few days, whereas the flu can cause an otherwise healthy person to miss work for an entire week or more.
Diagnosing one or the other is often done on the basis of clinical history, symptoms, and experience. While you can test for the flu virus, there is no test for cold viruses, so physicians will usually use your clinical history and symptom presentation to make a diagnosis. Expect your doctor to ask about how long your symptoms have been around, in what sequence they came about, and how they've evolved. Additionally, your physician may take the time of year into account when making a diagnosis. For example, there might be a burst of people coming down with colds in September, but if it's January or February and someone comes in with a bad headache, high fever, or abrupt onset symptoms, then the likelihood of them having the flu is higher.
Because the viruses that cause colds are different from the influenza virus, a cold cannot turn into the flu. However, if you get particularly unlucky, you could technically have both viral infections at the same time. There’s also a key difference between bacterial infections, like a sinus infection, and viral infections like a cold or the flu. Sinus infections can surface as a secondary complication of a cold, but they are caused by bacteria rather than viruses. If a person with a head cold (we distinguish between a head cold and a chest cold simply by where the cold manifests) had inflammation that lasts long enough for a bacterial infection to develop in the sinuses, they could wind up with a sinus infection.
However, since complications of the flu tend to localize in the lungs more than the upper respiratory tract, sinus infections don’t usually surface as secondary complications of the flu. In cases of the flu, pneumonia is a more common complication. Because of this risk, as well as the severity of its symptoms and longer duration time, the flu is more dangerous than a cold. Cold viruses will not land someone in the hospital, while the flu can escalate to that and have a much higher complication rate.