Fever: when to treat it, and when to leave it alone

A raised temperature can help your body fight bugs, but it can also be dangerous. Here’s how to tell the difference.

Carmel Sparke
April 2019

Even though fevers can seem scary, especially in children, they can be beneficial in warding off some bugs and illnesses. So what exactly is a fever, what causes it and when should you worry?

What is a fever?

Your body temperature is a measure of how warm your bloodstream is. Your body likes to stay within a fairly constant temperature of around 37°C, although this can vary a little from person to person, and at different times of the day and month.

Infectious diseases expert and paediatrician Professor David Isaacs says there's some debate around the exact temperature that indicates a fever, but most people agree it’s above 38°C.

What causes fever?

The most likely cause of a fever is your body fighting a virus, like a cold or gastro, although it can also be due to a bacterial infection like tonsillitis or a urinary tract infection.

Other reasons your body’s temperature may climb include: hormonal changes (like when a woman is ovulating), chronic illness (like rheumatoid arthritis) or tropical illnesses (like malaria), heat stroke, tumours or after taking some drugs.

“Your body's response to any infection is, ‘I don't like this bug, what am I going to do about that?’ and your immune system kicks in trying to fight it,” says Prof Isaacs.

Part of the body’s immune response involves the release of proteins called cytokines from the immune cells, and these raise your temperature.

Now what? Should you try to lower your temperature?

Painkillers like paracetamol and ibuprofen can help to reduce fever. But unless you have symptoms like a headache, or your child is irritable and miserable, there may be no need to treat a mild fever with these medications, says Prof Isaacs.

In fact, there’s a fair amount of evidence that the fever response may help fight off infection, in part because the immune system’s proteins and enzymes operate better in a warmer environment, he says.

Plus, some viruses don’t like heat, so there’s a chance a raised body temperature helps to kill them off.

But, unfortunately, higher temperatures don’t fight all bugs, they can cause uncomfortable symptoms like headaches and if they get too high, they can be dangerous.

When should you worry?

Children may get several respiratory viral infections a year, so if you’re a parent or caregiver, keep a thermometer handy.

Temperatures above 40°C can be concerning, especially in kids under 5, as they can trigger febrile convulsions or seizures.

The Royal Children’s Hospital Melbourne recommends you get medical advice if your child:

• has a fever above 38°C, especially if they’re feeling unwell
• has had a fever for more than 2 days
• is under 3 months and has a fever above 38°C (they need to go to hospital even if they have no other symptoms).

Adults should seek medical attention if your temperature is above 40°C or has lasted for a few days.

But in general, fevers pass in a few days, once your body has fought off the infection and the levels of cytokines start to drop.