You’re feeling unwell and your body temperature is rising. As you retreat to bed and ponder your misery, thoughts start going through your head.
“I’ve been crook before without a fever. What’s different this time? Could it be that my cold — a virus — has developed into a secondary sinus or chest infection — bacterial — or have I picked up something more serious?”
Before you know it, you’ve moved onto thinking “maybe I need antibiotics … I’ll call the doctor in the morning”.
- Q: Does having a fever mean your infection is bacterial rather than viral?
- A: A fever usually means you have an infection but it doesn’t tell you if it’s bacterial or viral.
But fever expert Doctor Paul Young suggests you stop for a reality check.
Yes, having a fever is a clear sign that something is going on inside your body, says Dr Young, a fever researcher and intensive care specialist at Wellington Hospital.
But the fever tells you little about what’s making you ill or how unwell you are.
“The [most] common thing that makes you have a fever, far and away, is an infection. So if you have a fever, basically it [almost always] means you have an infection,” Dr Young said.
But both bacteria and viruses can cause fevers and there’s no specific difference between a fever caused by bacteria and one caused by viruses.
Some of these processes result in chemicals called pyrogens flowing into your blood stream.
When these pyrogens travel to the brain, they interact with a brain region called the hypothalamus, which acts as the body’s thermostat.
When the hypothalamus detects the pyrogens, it “resets” the thermostat to a higher temperature and tells the body to start processes that generate and retain more heat. The result is a fever.
Dr Young says that as well as pyrogens, nervous system signals can also trigger a fever.
“You do produce substances in your liver that act through the vagus nerve (a nerve connecting the abdomen to the brain) to change the thermostat,” he said.
But both bacteria and viruses can trigger the resetting of the body’s thermostat through these kinds of mechanisms.
So the presence or absence of a fever isn’t a good guide to whether it’s one kind of infection or the other.
You only have to be laid low with flu, which is caused by the influenza virus, to know that viruses can certainly cause fever.
Whatever the cause, it’s thought the increase in body temperature may be helpful in fighting the infection because most bacteria and viruses can only survive in a narrow temperature range.
By making the body hotter, it may help kill off the infectious organism. But no-one knows this for sure, Dr Young says.
Bacteria or virus — why does it matter?
The main reason you might want to know if you have a bacterial rather than a viral infection is because only the bacterial variety will respond to antibiotics.
Taking antibiotics won’t help you fight off a virus as antibiotics target and kill bacteria.
It’s also worth noting that antibiotics aren’t always necessary even if you do have a bacterial infection so it’s probably best to talk to your doctor about this.
It’s important we all understand this as taking antibiotics when we don’t need them can contribute to antibiotic resistance, which makes it harder to find drugs that work when you do have a serious bacterial infection that needs treating.
The most common pathogens that cause infections are actually viruses, Dr Young says. And by and large viruses are self-limiting illnesses that get better by themselves with time, without you needing to do anything much, other than perhaps rest.
If you do happen to have a viral infection that’s more serious, other symptoms are more important tell-tale signs of how ill you are, he says.
“If it’s an exception [to the ‘self-limiting’ virus rule], it’s not going to be the fever that’s telling you it’s a bad one,” Dr Young said.