Everybody in the UK has been asked to stop non-essential contact with other people and avoid all unnecessary travel. This is known as social distancing.
It follows people with flu-like symptoms being asked to self-isolate at home, to avoid infecting others.
So, what do you need to know about social distancing and self-isolation?
What is social distancing?
Social distancing means trying to avoid contact with other people.
It means spending less time in public places, where a lot of people are around.
Within days, it expects to announce measures for people in at-risk groups to stay at home for 12 weeks. This affects pregnant women, people aged over 70 and those with underlying health conditions.
What is self-isolating?
Self-isolating means cutting yourself off from the rest of the world.
From now on, if one person in a household starts to display flu-like symptoms - defined as a fever of above 37.8C or a persistent cough - everyone living there must stay at home for 14 days.
He said that meant avoiding leaving the house "even to buy food or essentials", adding people could go out to do exercise, but only at a safe distance from others.
The person with the symptoms should stay in a well-ventilated room with a window that can be opened, and keep away from other people in the home.
They should ask for help for groceries, other shopping or medication, which can be dropped off on the doorstep by friends, family or delivery drivers.
People are being advised not to ring NHS 111 or their GP to report their symptoms unless they are worried.
The Covid-19 disease can cause a fever, cough and breathing problems. It takes five days on average for people to start showing the symptoms.
What if someone self-isolating shares a home?
If someone is self-isolating and shares a kitchen, they should try to avoid using it when other people are there and take meals back to their room to eat. Clean all the surfaces at home with household cleaning products daily.
Although they might not be able to entirely separate themselves from family members or flatmates, the advice is to limit contact as much as possible.
If possible, stay at least 2m (6ft) from other people they live with and sleep alone. Keep away from vulnerable people.
People living with someone in isolation should wash their hands often, using soap and water for at least 20 seconds - especially after coming into contact with them.
Other people shouldn't share towels, toiletries or other household items with someone in isolation, who should have a separate bathroom. If that is not possible, the isolated person should use the bathroom last, cleaning it thoroughly afterwards if they are able.
Any rubbish that the isolated person has been in contact with should be double-bagged and kept.
Will I still get paid?
Workers will get statutory sick pay from the first day off work, not the fourth, to help contain coronavirus, the prime minister has said.
This means that people who have to self-isolate or who are ill would get an extra £40 to cover the first four days. This would take it up to its usual rate of £94.25 per week.
Many casual or agency workers may also be entitled to sick pay but self-employed people are not. Citizens Advice says people on zero-hours contracts can still get sick pay and should ask their employer.
In the meantime, the government says you can claim universal credit or employment and support allowance if you're prevented from working for public health reasons.
If you need to care for a relative or your child's school is closed and you need to look after them at short notice, your employer must give you time off, but it may be unpaid.