Ask the doctors: Cold and flu: different viruses with similar symptoms

Dear Doctor: When my husband and I both got sick last month, he was diagnosed with a cold and I was diagnosed with the flu. What’s the difference?

Dear Reader: Both a cold and the flu are respiratory illnesses caused by a virus. The difference between the two is the type of virus that is involved.

The flu is caused by the influenza virus, of which there are four types – A, B, C and D. The seasonal epidemics that make millions of people miserable each year are caused by human influenza A and B viruses. The type C influenza virus typically results in a shorter and milder illness and is not associated with epidemics. Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people.

The influenza A virus is further broken down into subtypes based on the presence of two proteins on the surface of the virus. These are hemagglutinin (H), which has 18 known subtypes, and neuraminidase (N), which has 11 known subtypes. To make things even more challenging for epidemiologists, many different combinations of the H and N proteins are possible. That means influenza A viruses can be further divided into different strains, like H1N1 or H3N2. Although influenza B viruses are not broken down into subtypes, they are divided by lineage and strain.

Many viruses can cause the symptoms of the common cold, but the culprit in more than half of all cases is the human rhinovirus. As with the influenza virus, viruses that cause a cold can hang in the air for up to several hours in the mist of microscopic droplets that are released when an infected person coughs or sneezes. Breathe them in, and you can become infected. Similarly, when you touch something that has been contaminated by a virus, like a door handle, dishware or even a piece of fruit in the produce section, and then touch the soft tissues of your eyes, nose or mouth, the virus can enter your body.

Viruses are basically bare-bones genetic factories that exist to hijack cells and force them to quickly reproduce the virus in great numbers. As your immune system becomes aware of the presence of the intruders, it mounts the defense that we recognize as the symptoms of a cold or the flu. This commonly includes a sore throat, mucus production, cough, sneezing, fever, headache and body aches. Each of these arises from the immune system’s efforts to create an inhospitable environment for the new virus copies that the body has been forced to produce, and to expel them from the respiratory system.

Because there is so much overlap in symptoms, it can be hard to tell the difference between a cold and the flu. A cold typically inches in, symptom by symptom. But the flu hits hard and fast, with headache, body aches, fever and exhaustion, each of them often severe. The flu is also more likely to lead to complications that require prescription medications, or when serious enough, can lead to hospitalization. The only way to definitively diagnose a case of the flu is through a laboratory test.