The UK recently loosened restrictions and now things are tightening up again.
Prime Minister Boris Johnson has called on the public to "summon the discipline and the resolve" to follow the new coronavirus rules announced on Tuesday.
In a television broadcast to the country he warned the government may go further if people do not stick to them.
New measures have been brought in across the UK, with Mr Johnson warning rules could last for up to six months.
In England, rules on face coverings were expanded and the number of people allowed at weddings has been halved.
Pubs, restaurants and other hospitality venues will have to close by 22:00 BST, while the fines for breaking the rules will also increase to £200 on the first offence.
Hospitality venues will also have to close early in Scotland and Wales - but Scotland has gone further, banning people from visiting other people's homes.
It comes as the number of UK cases rose by 4,926 on Tuesday, government figures showed, with deaths increasing by 37.
Mr Johnson said he was "deeply, spiritually reluctant" to infringe on anyone's freedom, but added: "unless we take action the risk is that we will have to go for tougher measures later".
"If people don't follow the rules we have set out, then we must reserve the right to go further," he added.
"And to those who say we don't need this stuff, and we should leave people to take their own risks, I say these risks are not our own," he said.
"The tragic reality of having Covid is that your mild cough can be someone else's death knell.
Mr Johnson said the "single greatest weapon" that the UK brings to the fight against the virus is the "common sense of the people themselves".
He added: "If we follow these simple rules together, we will get through this winter together. There are unquestionably difficult months to come.
"And the fight against Covid is by no means over. I have no doubt, however, that there are great days ahead.
"But now is the time for us all to summon the discipline, and the resolve, and the spirit of togetherness that will carry us through."
What are the new rules?
Office workers are being told to work from home again if possible,
Penalties for not wearing a mask or gathering in groups of more than six will increase to £200 on the first offence
From Thursday 24 September, all pubs, bars and restaurants will be restricted to table service only. Takeaways can continue
Also from Thursday, hospitality venues must close at 22:00 - which means shutting then, not calling for last orders (in Scotland the same curfew rule comes into force on Friday)
Face coverings must be worn by taxi drivers and passengers from Wednesday
Retail staff and customers in indoor hospitality venues will also have to wear masks from Thursday, except when seated at a table to eat or drink
From Monday 28 September, only 15 people will be able to attend weddings and civil partnerships, in groups of six. Funerals can still take place with up to 30 people
Also from 28 September, you can only play adult indoor sports in groups of less than six
The planned return of spectators to sports venues will now not go ahead from 1 October
People across Scotland are being advised not to visit other households indoors from Wednesday 23 September onwards. This will become law from Friday.
There will be exceptions for those living alone, or alone with children, who form extended households. The rules will also not apply to couples who do not live together, or to tradespeople or for the provision of informal childcare - such as by grandparents
From Friday, pubs and restaurants will have to close by 22:00
The first minister urged people not to book overseas travel for the October holiday
Pubs, cafes and restaurants in Wales will have to close by 22:00 from Thursday - and sales of alcohol from off-licences and supermarkets will also be stopped after that time
Pubs will also be required to provide table service only
In Northern Ireland:
From Tuesday evening, people cannot meet anyone who they don't live with inside their home
There are only limited exceptions.