It’s the wonder of the modern medical age: You catch the flu, pop an antibiotic and feel better… right? Wrong — when to take antibiotics depends on whether you’ve got a bacterial or viral infection. But the belief that antibiotics can treat any trivial ailment is  so common that in India, antibiotic use shoots up in the July to September flu season, according to a 2014 study in The Lancet.

Yet antibiotics are completely ineffective against the flu, and a host of other viral illnesses. The wonder drugs are only effective against bacterial infections.

A quick refresher in biology

Most infections are caused by either viruses or bacteria, two very different types of microbes.

Bacteria are single-celled organisms that multiply by the process of simple division. They can thrive in many environments, including inside humans, and many strains are beneficial to healthy body functioning. But many more are infectious and make us sick. Common illnesses, from an ear infection to bronchitis, strep throat or a urinary tract infection, are caused by bacteria, as well as more serious illnesses like typhoid, leprosy and tuberculosis.

Bacteria were first observed as far back as 1676 by a Dutch scientist, though there was nothing the world could do about them then. It wasn’t until 1928 that Alexander Fleming invented penicillin, the first antibiotic, credited with saving millions of lives. But as bacterial illnesses became something people could easily live through, the world was only starting to have a grasp of what a virus was.

Viruses are roughly 100 times smaller than bacteria, and can only survive inside a host. In fact, viruses can infect bacteria that, in turn, infect us, giving our immune systems double the trouble. Like bacteria, we often have symbiotic viruses in us already, that do us no harm. But viruses are also responsible for an equally wide variety of ailments, from the common cold (rhinovirus), the flu (influenza), and chicken pox, to HIV/AIDS and Ebola.

A handful of illnesses – pneumonia, meningitis and diarrhoea – can be caused by either bacterial or viral infections.

When to take antibiotics

Unlike bacteria, however, when a harmful virus infects us, antibiotics are totally ineffective.

This is because antibiotics work by inhibiting bacteria’s ability to grow and replicate. But viruses don’t proliferate in the same division process as bacteria. Viral infections spread in the body by adding their DNA to our own. In other words, antibiotics are made to stop something that viruses don’t even do. Treating a viral infection with antibiotics is like shovelling the streets of Chennai clear of snow.

Unfortunately, there is no group of drugs that cure viruses in the same way antibiotics cure bacterial infections. When it comes to day-to-day ailments, that’s a frustrating reality.

“Most viral colds are self-limiting, and doctors prescribe medicines to provide symptomatic relief,” says Dr. Atul Ahuja, consultant ENT specialist with the Indraprastha Apollo Hospital in New Delhi.

Doctors may also provide immune-boosting medications, to prevent a bacterial infection from setting in on top of the viral one, he says. But that’s all doctors – or drugs – can do.

Why it matters

Understanding the difference between when to take antibiotics depending on what they can and can’t treat is critical to our health. Many times, it’s virtually impossible to identify immediately the difference between a bacterial and viral infection; expensive blood tests that take at least 24 hours are the only way to be sure right now (though that may soon change). So most doctors will treat symptoms for five to seven days after onset until the difference – and course of treatment – become clearer.

“When people don’t see much improvement in a couple of days, or experience deterioration, they don’t understand they are under observation,” Dr. Ahuja says, but it’s an important waiting period.

He’s seen an increasing impatience among patients, and indeed, demand for instant relief – by way of antibiotics before the cause of infection is clear – is growing worldwide. And doctors are giving in.

Dr. Ahuja says doctors who acquiesce typically do it out of concern for their practices.

“[Patients] move on to a new doctor, who is in a better position to understand if it’s a viral or bacterial infection because the illness has progressed,” he says. “And by then the first doctor earns a bad name.”

But this combination of self-medicating and prescribing unnecessary antibiotics is coming at a price: drug-resistant bacterial diseases are becoming rampant and threatening to undo all the good health that started with penicillin nearly 90 years ago.

For this reason, Dr. Ahuja cautions against self-medication and urges patients to trust their doctors and have patience.

Easier said than done, when you’re feeling miserable. But knowing how symptoms differ between common bacterial and viral infections can help you make peace when you need to wait out a viral infection — and know when to seek a doctor’s care.