Sneezing? Runny nose?

Those symptoms are typically the same whether you are in the throes of seasonal allergies or in a battle against a viral infection.

Allergies are not contagious unlike a viral illness, which is contagious, making it hard to determine when to isolate from others to avoid spreading the infection.

So, how do you know when it's time to pop an antihistamine and soldier on to work or school, or raise a white flag and take a much-needed sick day?

It can be hard to tell.

"Both of them involve your immune symptoms being activated," says Michael Baker, a professor of public health at the University of Otago who influential in New Zealand's Covid response.

"Certainly with a viral infection, a lot of the symptoms are from the immune response, not the infection."

Professor Michael Baker Photo: Luke Pilkinton-Ching

Whether it's allergies or sickness, your white blood cells and certain molecules are being called up for service.

Here's how to tell what the heck is going on in your body:

Look for patterns

Year after year, "people would get familiar with the difference," says Baker.

Pay attention to the time of year and what potential triggers might be - like pollen, animal, household dust from dust-mites, he said.

A family history of allergies is also a place to look, advised Dr Luke Bradford, a GP in Tauranga.

"If a family is full of hay-fever and asthma and eczema, the chances are the kid is going to have it. There will be a link that a child in that family is going to have some kind of allergy."

"Similarly, if your mum's had a nasty bug and felt horrible last week, then families are likely to get [the bug] the next week."

Test it with an antihistamine

"Taking an antihistamine is a really good attempt because normally within half an hour, you'll know if it's making you feel better," says Bradford.

Dr Luke Bradford. Photo: supplied

You can get a basic antihistamine over the counter from your pharmacy.

Doctors can provide prescriptions for stronger stuff.

People do tend to get used to whatever antihistamine they are taking regularly so Bradford advises patients to mix it up every six months.

Pay a visit to your pharmacist or doctor - if you can get an appointment.

Do you feel unwell?

This might seem a little obvious, but a viral infection will likely have you feeling tired, lethargic and kinda yuck, says Bradford.

"Those people tend to feel ill and often it will come with a cough, fever, achy muscles."

That's not to say allergies can't be debilitating.

When they are severe, allergies can cause a drop in productivity or leave you feeling so crappy that you do take a sick day.

Snot colour and consistency

What a lovely subject.

If your snot is yellow and thick, it is more likely to be a viral infection so consider staying home from work, Bradford says.

"Whereas hay fever mucus is classically clear and watery."

Recently moved?

I had very little to do with seasonal allergies before I moved to New Zealand five years ago.

Now, I find myself waylaid for weeks during Spring and Autumn pondering the question this article is answering. Turns out it's a thing.

Moving cities, regions and countries can trigger new allergies or dial up the severity of existing seasonal allergies, explains Bradford.

"We get heaps of people coming to the Bay of Plenty having never had hay fever and suddenly you're like 'Oh my goodness.'"

"That's one of the lovely things about travel, isn't it? You go and you see different plants... but people won't have had huge levels of exposure to a certain type of pollen or grass..."

A new job can mean exposure to different amounts of dust or different allergens, bringing on an immune response.

The number of allergens in the air can change from year to year, making your immune symptom response more intense.

What to do?

If you think you've got allergies, chat with your pharmacist and then your doctor on what you can take to relieve the symptoms.

Resting, drinking lots of fluids, simple painkillers and a decongestant if necessary is the frontline response for a viral infection.

This is what sick days are for, if you're lucky enough to have them.

Antibiotics won't help unless you can't kick a chest or sinus infection.