Coronaviruses (CoV) are a large family of viruses causing illness ranging from the common cold to more severe diseases such as Middle East Respiratory Syndrome (MERS-CoV) and Severe Acute Respiratory Syndrome (SARS-CoV).
A pandemic - according the the WHO - is the worldwide spread of a new disease, as opposed to an epidemic, which is a disease contained within one country.
"Pandemic is not a word to use lightly or carelessly," WHO's Director-General Tedros Adhanom Ghebreyesus said.
"It is a word that, if misused, can cause unreasonable fear, or unjustified acceptance that the fight is over, leading to unnecessary suffering and death."
What are the symptoms of Covid-19?
Since these symptoms are those for a variety of illnesses, such as commons colds and flu, if you have them you do not necessarily have Covid-19.
Public Health England (PHE) said the virus may progress to more severe illnesses, such as pneumonia.
Older patients and those with underlying health conditions are more likely to progress to severe disease.
Are there any other symptoms?
Those living with someone showing these symptoms should also self-isolate, but for 14 days rather than seven - even if they are not showing any symptom of any kind.
This move came after a major study, published by Professor Tim Spector at King’s College London, found that people with a positive test result were three times more likely to report loss of smell and taste as a symptom than those who went on to test negative.
Prof Van-Tam told reporters it would mean 93% of cases where people have symptoms are now picked up, a rise from 91% previously.
What should you do if you think you may have Covid-19?
Shelley Folkes, a locum nurse, told ITV News "plenty of rest, lots and lots of fluid" are needed as well as "simple paracetamol to combat a fever."
Nichola Mazur, an NHS nurse who works on the frontline, said most who develop symptoms will not need to go to hospital.
"No, if you do have symptoms you don't go into hospital - if you do have the virus this can then spread to other people," said Ms Mazur.
"For most people, you will recover at home without the need for any medical intervention."
"Obviously, if your symptoms become very severe like extreme shortness of breath or reduced conscious level then you will need to call 999 and the ambulance service will assess you over the phone."
Treatment for coronavirus
There is currently no vaccine for Covid-19, though a number of potential vaccines are in development.
The virus attacks and kills two specific types of cells in our lungs, known as the goblet and ciliated cells.
The cells are vital to keeping the lungs healthy, losing them can lead to potentially lethal pneumonia. In some cases, the virus causes the immune system to go into overdrive, attacking the lungs themselves.
The virus can also attack the kidneys - and, in some cases, the blood vessels and circulation.
Antibiotics do not help, as they do not work against viruses.
Treatment instead aims to relieve the symptoms while your body fights the illness.
How is Covid-19 spread?
Coronavirus can spread from person-to-person in close proximity, similar to other respiratory illnesses, such as the flu.
Droplets of bodily fluids - such as saliva or mucus - from an infected person are dispersed in the air or on surfaces by coughing or sneezing.
These droplets can come into direct contact with other people or can infect those who pick them up by touching infected surfaces and then their face.
According to scientists, coughs and sneezes can travel several feet and stay suspended in the air for up to 10 minutes.
It is not yet known how long the virus can survive outside a host but, in other viruses, it ranges from a few hours to months.
Transmission is of particular concern on transport and in public places, where droplets containing the coronavirus could pass between people or via surfaces.
The incubation period of the coronavirus - the length of time before symptoms appear - is between one and 14 days.
It is thought Covid-19 can be transmitted before symptoms appear, making it easy to spread.
Who is particularly vulnerable?
A raft of new measures on shielding and protecting to help the nation's most vulnerable have been announced by the government.
Women who are pregnant with significant heart disease are also included on the list.
What precautions should we take?
Cover your mouth and nose with a tissue or your sleeve (not your hands) when you cough or sneeze and immediately bin used tissues
Wash your hands with soap and water often – use hand sanitiser gel if soap and water are not available
Avoid close contact with people who are unwell
Don't touch your eyes, nose or mouth if your hands are not clean
What do I need to do?
Avoid contact with someone who is displaying symptoms of Covid-19 (remember, these symptoms include high temperature and/or new and continuous cough)
Avoid non-essential use of public transport when possible
Work from home, where possible
Remain at home as much as possible
Do not meet up with friends and family - keep in touch using remote technology such as the phone, internet, and social media
Use telephone or online services to contact your GP or other essential services
Where did coronavirus come from?
On December 31 last year, the World Health Organization (WHO) was informed of a cluster of cases of pneumonia of unknown cause in Wuhan City, Hubei Province, China.
On January 12, Covid-19 was identified in samples obtained from cases.
Initial analysis suggested that this was the cause of the outbreak and it is thought to have originated from an illegal wildlife market, possibly from a pangolin (a scaly anteater which is prized in China for its use in traditional medicine).