A 21-year-old woman with no underlying conditions has died after contracting coronavirus, according to her family.
Chloe Middleton, from High Wycombe, Buckinghamshire, died last week.
In a Facebook post, her aunt Emily Mistry said Ms Middleton "had passed away from Covid-19" and urged others to "do your bit" to stop the spread.
The BBC understands that because Ms Middleton was not admitted as a patient to the hospital she was not tested for coronavirus.
According to government figures, most patients who have died with coronavirus had pre-existing medical conditions.
There have been concerns that younger people were ignoring warnings over its spread, believing the contagion was only a danger to the elderly.
But in a series of Facebook posts, Ms Middleton's family urged the public to heed safety advice and take the virus "seriously".
Her mother, Diane Middleton, wrote: "Please think again.
"Speaking from a personal experience, this so-called virus has taken the life of my 21-year-old daughter."
Ms Mistry added: "My beautiful, kind-hearted 21 year old niece has passed away from Covid-19.
"She had no underlying health conditions."
She said the family was "shattered beyond belief".
"The reality of this virus is only just unfolding before our very eyes," she added. "Please, please adhere to government guidelines.
"Do your bit. Protect yourselves and protect others. The virus isn't spreading, people are spreading the virus."
Ms Middleton's sister, Amy Louise, added that it was "about time people took this seriously, before too many people end up in this devastating position".
An 18-year-old man who died in hospital at the weekend after testing positive was being treated for "significant underlying health issues".
How deadly is the coronavirus?
The elderly and the unwell are more likely to die if they contract coronavirus.
But even among older people, "the great majority will have a mild or moderate disease", according to the UK's chief medical advisor, Prof Chris Whitty.
He also pointed out some young people have ended up in intensive care, and should therefore not dismiss it as a "trivial infection".
Current estimates from Imperial College London are that the death rate is almost 10 times higher than average for those over 80, and much lower for those under 40.
As well as age, those with diabetes, high blood pressure or heart or breathing problems are also deemed more at risk.
Read more: How deadly is the coronavirus?
Update 1 April 2020: This story was updated to make clear that Ms Middleton was not tested for coronavirus.