The most common symptoms of a coronavirus infection are fever (temperature above 37.8C or 100F), dry cough, difficulty breathing or shortness of breath, and fatigue.

There is significant overlap with fluids from the flu or the common cold, but other symptoms are almost unique to COVID-19, the respiratory illness caused by the SARS-CoV-2 coronavirus.

For instance, doctors have noticed that some cases experience anosmia — a complete loss of smell — as well as the loss of taste. These two symptoms, when they appear, tend to show in the early stage of infection. Some might not develop other symptoms of COVID-19 other than anosmia.

To make matters more confusing, many patients show no symptoms at all, despite producing viral loads in their respiratory tract which can spread to other people who may be much more vulnerable to developing life-threatening symptoms.

How long does it take to show symptoms?

According to scientists, it takes around 5 days on average for an individual who has just been infected in order to show symptoms. However, symptoms may take up to 14 days to surface in some cases, which is why many local authorities advise that people self-isolate for at least two weeks after returning home from abroad or from some region where the pandemic is evolving rapidly.

There is still a lot we don’t know about the virus, but public health experts believe that most people who get infected will go on to develop only mild symptoms of COVID-19.

However, the disease can prove to be life-threatening for some vulnerable groups, typically the elderly and those with preexisting medical conditions ( asthma, diabetes, heart disease, high blood pressure, autoimmune disease, cancer).

Scientists believe that infected individuals are the most contagious when they have obvious symptoms, such as fever and cough.

How to tell apart COVID-19 symptoms from the flu or allergies?

COVID-19 and the common cold have some overlapping symptoms. These include a sore throat, fatigue, and a cough.

One of the most reported differences between a common cold and coronavirus is a runny nose. This is common for a cold but rare for COVID-19. Similarly, a fever is possible with a cold but is rare. A fever is common with coronavirus.

Shortness of breath is common with coronavirus but rare for a common cold. The allergy symptoms that overlap with COVID-19 are nasal congestion, cough, runny nose, and sore throat.

“The main difference that people should remember is with fevers,” said Dr. Alex Thomas, an asthma and allergy specialist at the Center for Asthma and Allergy in Illinois. “That is something you would only really see with a viral infection such as COVID-19. Even though the term is ‘hay fever,’ you would not get a fever with environmental allergies.”

However, the only to tell for certain if you have COVID-19 is to take a test.

How many cases show no symptoms?

SARS-CoV-2 is particularly contagious due to its considerably high incubation period, lasting 5.1 days (median) but which can be up to 14-16 days. During this time, the infected person can transmit the virus to other people oblivious to the fact that she is actually sick.

A study performed by researchers at Hasselt University in Belgium found that 48% of the transmission in Singapore and 62% in Tianjin, China, was enabled by people who had yet to develop symptoms.

It has become increasingly clear that a large part of COVID-19 cases never shows any symptoms, but it is what is the rate of asymptomatic cases. Such estimates are difficult to ascertain while an epidemic (or pandemic in this case) is still in progress.

Two new studies offer some hints, though. The first was published in the journal Eurosurveillance by an international team of scientists from Japan, the U.S., and the UK, who analyzed COVID-19 cases aboard the Diamond Princess, which was docked in Yokohama, Kanagawa Prefecture in Japan.

The cruise ship had 3,711 onboard, which were immediately put under quarantine (they weren’t allowed to set foot ashore) after one of the guests was tested positive. Eventually, 696 passengers and crew members tested positive and six died of the illness.

Using a statistical model that filled some of the gaps in the reported data because not all passengers had been tested, the authors of the new study estimated that 17.9% of those infected were asymptomatic.

This asymptomatic rate is similar to that of the flu virus, the authors wrote in their study. I’ll take this opportunity to note that while the two types of viruses might share a similar asymptomatic rate, that’s not to say that the novel coronavirus is like influenza. SARS-CoV-2 is both different and a lot more dangerous than the virus for the flu.

A second study published in the International Journal of Infectious Diseases analyzed the rate at which symptoms showed in 565 Japanese nationals who had been evacuated from Wuhan. A third of those who had tested positive for SARS-CoV-2 (13 people in total) were asymptomatic cases — a much higher rate than found among Diamond Princess passengers and crew, but the confidence interval is much wider (7.7% to 53.8%). There were only 9 symptomatic cases and 4 asymptomatic cases.

Both studies have their limitations. The study on the Japanese nationals included a very small sample size of confirmed cases while the one that monitored individuals from the Diamond Princess is not very representative of a wider demographic. However, these preliminary results are still helpful in establishing a rough estimate for the transmissibility of SARS-CoV-2.

Perhaps the most reliable study of COVID-19 asymptomatic involved a country-wide experiment: the situation in Iceland. The island state has a population of only 364,000 people and is extremely isolated. It also tests a lot per capita compared to other countries.

Of the 10,000 tests or so carried out in Iceland so far, which yielded 473 positive cases. Of these, half were non-symptomatic. Similarly, a survey carried out in the Italian town of Vo, which has a population of 3,300, reported similar results showing that about 50% of all confirmed cases of infection were non-symptomatic.

Similar studies carried on Chinese patients report similar things: the virus is spreading far more than we are anticipating. For every known case, there are five or ten people whose symptoms go undetected.

“Covid-19 is proving to be a “stealth virus” in that we now know a significant amount of transmission is through people who don’t have symptoms. The exact % will depend on setting, but is high enough to make testing and surveillance key tools,” said Dr. Jonathan Quick in a Reddit AMA.

On top of those showing no symptoms of disease despite being infected and capable of spreading the virus, there are also many mild cases that can be easily dismissed as a common cold or flu.

When to see a doctor

If you have COVID-19 symptoms or you’ve been in contact with someone diagnosed with COVID-19, contact your doctor or clinic right away for medical advice on how to proceed further with testing. Be careful not to leave anything out when contacting healthcare providers, such as when symptoms first started to appear or who you’ve been in contact with recently.

If you have emergency COVID-19 signs and symptoms, seek care immediately. Emergency signs and symptoms can include:

• Trouble breathing
• Persistent chest pain or pressure
• Confusion
• Blue lips or face