A "tidal wave" of flu infections has led to a 70% increase in hospital cases in England in just seven days, the NHS has said.
An average of 1,861 patients with flu were in hospital every day last week - up from 1,099 in the previous week and 402 at the same time last year.
Flu cases are currently highest among 5 to 14-year-olds, according to the UK Health Security Agency, but adult cases are expected to rise when schools and nurseries close.
Cases of norovirus (the winter vomiting bug) and RSV (a common cause of coughs and colds, and a cause of chest infections in babies) are also on the rise.
NHS England said an average of 837 beds were occupied last week by people with norovirus symptoms - up 10% on the previous week and 64% on last year.
Meanwhile,152 children were in hospital with RSV each day - up from 142 the previous week and 107 last year.
COVID patients took up 1,343 beds, down from 1,390.
Read more on 'quad-demic':
What is it - and how you can protect yourself
The mix of different illnesses has sparked fears of a "quad-demic" this winter.
It refers to the "four very common viral illnesses that circulate every winter in different peaks," GP David Lloyd told Sky News Breakfast.
The risk and complication rate of people catching all four at once heightens at this time of the year as people are spending more time indoors with friends and family.
Vaccination drive
Eligible people are urged to get vaccinated as soon as possible to avoid picking up a bout of festive flu.
The NHS provides vaccinations against flu, COVID and RSV.
Over-65s, pregnant women, and people with certain health conditions are among those who can get a free flu vaccination at a pharmacy.
Jabs are also being administered at places such as Christmas markets, football clubs, and supermarkets.
RSV vaccines are recommended during pregnancy - to protect the baby when it's born, and for adults aged 75 to 79.
Professor Sir Stephen Powis, the boss of NHS England, said: "The tidal wave of flu cases and other seasonal viruses hitting hospitals is really concerning for patients and for the NHS - the figures are adding to our 'quad-demic' worries."
He added: "With one week left to book your vaccine, I cannot stress enough the importance of getting booked in to protect yourself against serious illness and to avoid 'festive flu'."
Health Secretary Wes Streeting echoed the call, urging people to "protect themselves, their family, and the NHS by getting vaccinated before it's too late".
Busiest-ever November for A&Es
Other NHS figures show the overall waiting list for treatment decreased for the third month in a row.
It went from 7.57 million in September to 7.54 million in October, after peaking at 7.77 million in September last year.
People forced to wait longer than Keir Starmer's long-term goal of 18 weeks also decreased slightly, from 3.14 million in September to 3.11 million in October.
The number waiting a year or more (234,885) was the lowest since December 2020 and down from a peak of 436,127 in March 2021.
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Meanwhile, it was the busiest November on record in A&Es in England, with 2.31 million attendances.
The number of patients being seen within the four-hour target slipped from 58.1% in October to 57% in November.
For ambulances, the average handover time was 42 minutes 59 seconds - longer than the 39 minutes 29 seconds in the same week last year.
Some 16.3% of handovers last week, or 14,672 patients, were delayed by more than an hour.
(c) Sky News 2024: 'Festive flu' warning: 'Tidal wave' of infections leads to 70% jump in hospital cases