The coronavirus outbreak has left many experts wondering how many people may catch the infection.
Virtually unheard of just two months ago, the Covid-19 strain has spread well beyond its epicentre in the Chinese city Wuhan.
More than 80,300 cases have been confirmed globally, of which 77,660 are in mainland China, according to John Hopkins University data. The death toll exceeded 2,000 on Tuesday.
In the UK, 13 cases have been confirmed, with health secretary Matt Hancock stating more are “expected”.
Amid fears the outbreak may become a “pandemic”, the World Health Organization stressed the term “does not fit the facts”.
A Harvard doctor later predicted between 40% and 70% of people all over the world could catch Covid-19 within the next year, with the vast majority of cases being mild.
Other experts have questioned this claim, stressing it is too early in the outbreak to predict how it may play out.
Is it possible to predict how many people may become infected with coronavirus Covid-19?
Plateauing infection rates have left some quietly optimistic Covid-19 may have “peaked” in China.
A change in how the virus is diagnosed caused cases to spike over night, from 44,700 on 12 February to 59,800 the following day.
Incidences have since been rising more slowly.
Mainland China had 77,200 confirmed cases on 24 February, increasing by 500 patients the next day.
Nevertheless, the infection is taking hold thousands of miles away.
South Korea is second to China, with 977 confirmed patients and 10 deaths.
Italy is faring worst in Europe, with 283 incidences and seven deaths.
To try and get a handle on the outbreak, scientists from all over the world are investigating how infectious Covid-19 really is.
Dr Marc Lipsitch - from Harvard - “predicts that within the coming year, some 40-to-70% of people around the world will be infected with the virus that causes COVID-19”, The Atlantic reported.
“But, he clarifies emphatically, this does not mean all will have severe illnesses”.
Other experts have questioned how much weight this prediction holds.
“These figures are calculations from the estimated basic reproduction number,” said Professor Christl Donnelly from Imperial College London.
This is the number of people an infected person goes onto infect.
A reproduction number of two would mean every patient is expected to pass the virus to two others.
“[These calculations] assume transmission continues at that rate,” said Professor Donnelly.
“Interventions bring the reproduction number down.”
The province Hubei, of which Wuhan is capital, has been under “lockdown” for more than a month.
Its almost 60 million residents have seen public transport restricted, shops shut and flights suspended.
Residents of Italy’s Lombardy and Veneto regions are also affected, with 11 towns under “quarantine”.
This is not the first time Covid-19’s reproduction number has been debated.
Scientists from the London School of Hygiene & Tropical Medicine (LSHTM) previously estimated the number fluctuated between 1.5 and 4.5 before travel restrictions were introduced in Wuhan, stressing “substantial uncertainty”.
This led them to predict the outbreak may peak in “mid-to-late February”.
Speaking at the time, Dr Robin Thompson from the University of Oxford, said: “One proviso must be forecasting the peak of any outbreak is challenging, and so there is significant uncertainty in estimates of both the timing of the peak and the total number of cases that will occur.”
Read more: How is the coronavirus Covid-19 diagnosed?
Despite ongoing research, Covid-19 remains somewhat mysterious.
“Transmission is a feature of the virus and that’s not understood,” said Professor David Heymann from the LSHTM.
“Models are estimations. They may be true today and not tomorrow.”
Professor Heymann described Professor Lipsitch’s prediction as an “upper estimation” of what could be to come.
Even when more is discovered about Covid-19, it may be difficult to classify the virus’ transmission, with rates varying according to the circumstances.
“Pathogens sometimes get stamped with a reproduction number,” said Professor Donnelly.
“It can vary depending on household size and in hospital settings.”
What is the new coronavirus Covid-19?
Covid-19 is one of seven strains of the coronavirus class that are known to infect humans.
Others range from the mild common cold to the severe acute respiratory syndrome (Sars) outbreak that killed 774 people in 2004.
Most of the people who initially became unwell worked at, or visited, the Wuhan market.
The only know method of transmission is face-to-face via infected droplets that have been coughed or sneezed out.
Symptoms tend to be flu-like, such as fever, cough and breathlessness.
In the most severe cases, victims develop pneumonia.
This comes about when a respiratory infection causes the alveoli (air sacs) in the lungs to become inflamed and filled with fluid or pus.
The lungs then struggle to draw in air, resulting in reduced oxygen in the bloodstream.
“Without treatment the end is inevitable,” said the charity Médecins Sans Frontières.
“Deaths occurs because of asphyxiation.”
While no one can say for sure where the virus came from, bats seem most likely.
The nocturnal creatures are thought to have been behind Sars and fellow coronavirus Middle Eastern respiratory syndrome (Mers), which killed 858 people in its 2012 outbreak.
Scientists from Peking University in Beijing suggested snakes may have been the “intermediate host” for Covid-19.
A team from South China Agricultural University later found it could have “jumped” from bats to humans via pangolins.
Covid-19 has no specific treatment, with care being “supportive” - like ventilation - while a patient’s immune system works to fight off the virus.
To prevent infection, the NHS recommends regular hand washing and avoiding those with suspect symptoms.