Is it COVID-19 or just a cold? It's a question many are asking themselves as fall and winter bring more cases of the common cold and seasonal flu. The symptoms may be hard to distinguish, given that all three conditions can result in a cough. But each has its own hallmarks.

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An August study from the University of Southern California identified a distinct order of symptoms among COVID-19 patients: Most symptomatic patients start with a fever, followed by a cough. For seasonal influenza, it's typically the opposite - people generally develop a cough before a fever.

If you get a common cold, meanwhile, that's more likely to start with a sore throat as the first symptom, according to the Centres for Disease Control and Prevention (CDC).

However, these symptom lists - and the order in which they arrive - aren't foolproof. Plenty of COVID-19 patients don't develop a fever at all, and some flu patients never come down with a cough.

That's why it's also helpful to consider how quickly symptoms appear and how long they last.

How COVID-19, flu, cold, and allergies manifest and progress

Coronavirus cases tend to develop more gradually than the flu. While some people start showing COVID-19 symptoms within two days of being infected, the disease's symptoms can take up to two weeks to manifest. On average, people start to feel sick five days after they were infected.

People with the flu, on the other hand, usually feel sick one to four days after exposure. Most patients then fully recover within less than two weeks, often as quickly as a few days.

Many coronavirus patients recover within two weeks as well, but a growing share of patients have reported symptoms that last for months.

Common cold symptoms, by contrast, usually reach their peak within within two to three days of infection - but, like the coronavirus, they often come on more gradually. Some cold symptoms last longer than others: Patients with a typical cold may have a sore throat for eight days, a headache for nine to 10 days, and congestion, a runny nose, or cough for more than two weeks.

Allergies tend to last longer - about two to three weeks per allergen - and won't resolve until the allergen leaves the air. Seasonal allergies also tend to be more severe in the spring, though.

The most common symptoms of each illness

Coronavirus cases run the gamut from asymptomatic to mild to severe.

"I've never seen an infection with this broad range of manifestations," Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told Facebook founder Mark Zuckerberg in July.

Many patients have reported conditions that don't appear on the official CDC list, including hair loss, hiccups, rashes, and purple, swollen toes.

A large share of COVID-19 patients lose their sense of taste or smell - this is perhaps the strongest predictor of a coronavirus infection, according to a June study from scientists at Massachusetts General Hospital and King's College London.

A Spanish case study similarly found that nearly 40 percent of patients with COVID-19 developed smell and/or taste disorders, compared to just 12 percent of patients with the flu.

Symptoms like a fever or headache could help rule out allergies or the common cold as well. People with colds, meanwhile, are more likely to develop a runny or stuffy nose than COVID-19 patients. And cold symptoms are milder overall.

One of the hallmarks of allergies - itchy eyes - isn't associated with any of the other three illnesses.

Ultimately, the best way to know if you have COVID-19 is to get a diagnostic test. Until results come back negative, people should stay home if they're feeling sick or were exposed to someone confirmed to have the virus.

Everyone should also get a flu shot to minimise the risk of overcrowding at hospitals.

"This will be, in my opinion, the most important flu season of our lifetimes," US Surgeon General Jerome Adams said at a September Senate hearing. "Less flu and fewer hospitalizations will help conserve precious healthcare resources."

This article was originally published by Business Insider.