Flu, influenza and pneumonia: Decoding the confusion

Published on 7 February 2025

Think you know what “flu” really is? It’s more than just a sore throat and a runny nose. Here’s what you need to know – and why the flu vaccine is your best line of defence.

“I’ve got the flu,” you say. But do you?

If your nose is running and you’re feeling under the weather, it might or might not be influenza.

The term “flu” is often used loosely, and many confuse it with other common ailments. Here’s how to tell the difference between a simple cold and the more serious influenza – which can lead to pneumonia.

Flu, influenza or pneumonia: Is there a difference?

Many people casually use the term “flu” to describe any viral illness, especially one that gives us a cough and cold. But here’s the thing – influenza and flu are the same. “Flu” is just the shorthand for “influenza”, which is a viral infection caused by a specific family of viruses that can cause severe symptoms and complications in some people.

Still, people often mistakenly call other illnesses – like the common cold – “the flu,” even though they aren’t related.

So, if you have flu-like symptoms but it’s not influenza or pneumonia, what is it? It could be a common cold – a mild viral infection causing a sore throat, congestion and runny nose. It’s far less severe than influenza.

True influenza is caused by the influenza virus and can lead to complications such as pneumonia.

Influenza and pneumonia: What’s the connection?

Pneumonia is a serious lung infection that can occur after a flu infection. While influenza is caused by a virus, pneumonia can be triggered by a variety of bacteria, viruses or fungi. The risk of developing pneumonia increases in certain groups, such as the elderly, children and those with preexisting conditions.

“Influenza can progress to pneumonia, particularly in vulnerable individuals. In many cases, it’s not just the flu virus at play – bacteria often follow, complicating matters further,” shared Prof Paul Anantharajah Tambyah, Senior Consultant at the Division of Infectious Diseases, Department of Medicine, National University Hospital (NUH), and Professor at the Yong Loo Lin School of Medicine, National University of Singapore (NUS School of Medicine).

Flu season? Not in Singapore

Unlike in countries with colder climates, influenza doesn’t follow a predictable flu season here in the tropics. It can strike at any time of the year, making vaccination year-round a smart choice.

Why the flu vaccine matters

Many people underestimate the flu, assuming it’s just a bad cold. But the truth is, flu complications – like pneumonia – can be life-threatening.

Prof Tambyah further explained, “Influenza can progress to pneumonia, particularly in vulnerable individuals. It’s not just the flu virus alone causing damage – bacteria often follow, complicating matters further.”

The common misconception: “The flu shot gave me the flu!”

No, it didn’t. The flu vaccine cannot give you the flu. It contains an inactivated or killed form of the virus that helps your body build immunity. If you feel unwell after getting vaccinated, it’s likely just your immune system doing its job.

Should you get vaccinated even if it’s not the flu?

Yes! Even if you've had a cold or another flu-like illness, those don’t offer protection against influenza – the real flu. And because the flu can lead to complications like pneumonia, the flu shot is still your best defence, especially if you or your loved ones have chronic medical conditions.

Stay protected, stay healthy

It’s easy to think of influenza as a mere cold, but it’s much more than that – and pneumonia is no minor threat either. A simple flu vaccine can shield you from the serious complications that may follow.

So, next time you say you have the flu – ask yourself: Do you really have the flu?

In consultation with Prof Paul Anantharajah Tambyah, Senior Consultant, Division of Infectious Diseases, Department of Medicine, NUH, and Professor, NUS School of Medicine.