Health leaders are warning that up to 8,000 people could be taken to hospital before a flu wave currently gripping the country reaches its peak.
A new flu virus is believed to be much more infectious than previous strains and has already led to a record number of patients needing urgent hospital care.
The NHS said that, unlike a cold, flu symptoms can come on very quickly and can include a sudden high temperature that appears within a few hours.
With a cold, a high temperature would appear gradually.
Flu can cause a headache, dry cough and aching body - while a cold mainly affects your nose and throat.
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The UK Health Security Agency (UKHSA) said it is seeing the flu among younger patients and fears the virus will now spread to older, more vulnerable patients.
Dr Jamie Lopez Berbnal, consultant epidemiologist at the UKHSA, told Sky News: "The new strain circulating has evolved a bit more rapidly than normal.
"And that could mean that there's a bit less immunity in the population from what we normally see in a flu season.
"And that could mean that the flu virus spreads a bit more than usual. And that's something that we've started to see already, in particular in children and young adults.
"But we're now starting to see flu take out a bit more in older adults, who tend to be more vulnerable to flu as well."
Professor Julian Redhead, national director for urgent care for NHS England, said this flu season would be a "big challenge" for the health service.
"I think it's going to be a big challenge for us as we go forward because it just increases the pressure.
"I mean, at the moment we've got around, say 1,500, maybe 2,000 patients in hospital with flu and the predictions are that it will rise to somewhere between 5,000 and 8,000.
"So you can see the pressure that will be put on the system will be very large and that's why we're concerned."
