A mysterious and severe form of hepatitis has been found in 200 children, predominantly on the European continent.
A mysterious strain of hepatitis has recently been identified in nearly 200 children across 12 countries, and at least one child has died of the severe disease.
To date, 190 cases of hepatitis with unknown origins have been reported, 140 of them in Europe.
In the UK alone, 114 children have fallen ill, and 10 have needed a liver transplant.
The Covid-19 vaccine is the first probable cause to come to mind during the pandemic. However, there is no relation between the rare form of the disease and the vaccine.
None of the patients under 10 years of age in the UK are known to have been vaccinated.
On April 24, European health officials said that they had not found a link between cases of a mysterious liver disease outbreak in children.
“So far, there is no connection between the cases and no association to travel,” said Andrea Ammon, director at the European Centre for Disease Prevention and Control in Stockholm.
Experts say the cases may be linked to a virus commonly associated with colds, but the research is inconclusive.
As the world tries to find out what is behind the mysterious and severe form of hepatitis, here is what we know about it so far.
Where did it emerge?
The first five cases were detected in Scotland on March 31 by "astute clinicians, realising they were seeing something unusual," said Meera Chand, director of clinical and emerging infections at the UK Health Security Agency.
However, these children did not have any of the five known hepatitis viruses, A, B, C, D and E and such cases are very rare, the doctor said.
In Scotland, four to five unknown hepatitis cases are seen in a year, Chand added.
After the UK, Spain had the next highest number of cases with 13, followed by Israel with 12 and the United States with nine. Small numbers have also been recorded in Denmark, Ireland, the Netherlands, Italy, Norway, France, Romania and Belgium.
What are the symptoms, and who is most affected?
Generally, children aged from one month to 16 years old have been affected, but most cases have been recorded in those under 10, with several cases in children under five.
Before the children showed signs of severe hepatitis, they had several symptoms such as jaundice, diarrhoea, vomiting and abdominal pain.
Doctor Chand outlined the possible signs of the mysterious hepatitis during an emergency session at the European Congress of Clinical Microbiology and Infectious Diseases (ECCMID) earlier on Monday in Portugal.
If parents see symptoms like a loss of appetite, stomach pain, dark urine, muscle and joint pain, high body temperature, itchy skin and grey-coloured faeces, Chand advises a visit to the doctor.
The main concern is the strain's severity, as approximately 10 percent of the confirmed cases had severe hepatitis and will require a liver transplant.
Hepatitis is an inflammation of the liver and is generally rare in healthy children.
Adenoviruses, common viruses that cause a range of illnesses like colds, bronchitis and diarrhoea but mostly do not lead to severe illness, were detected in 74 of the cases, the WHO said.
Adenoviruses have been found in every three of four cases in the UK.
Chand said the "leading hypothesis" was a combination of a normal adenovirus along with another factor making it more severe.
One possibility is that young children who have spent their "formative stages" under Covid measures such as lockdowns and mask-wearing over the last two years had not built up immunity to these adenoviruses.
Adenovirus rates in the UK plunged during the early stages of the pandemic but have spiked far above previous levels since measures were lifted, Chand said.
The WHO said that an "unexpected increase" in adenovirus cases has been recorded in several other countries recently, including Ireland and the Netherlands.
Other possible causes for the unknown strain could be a combination of adenovirus and Covid or previous Covid infections, Chand added.