At the school in Växjö, one in three – 208 out of 580 pupils – are currently staying at home due to illness, reports regional newspaper Kvällsposten and public radio broadcaster P4 Kronoberg.
According to the school, the influenza virus is the most likely culprit.
“In all my years I have never had so many pupils catch it so fast,” school nurse Kristina Lekberg told the newspaper.
Swedish health authorities advise anyone who catches the flu to stay at home and recover, but to contact your doctor's surgery if you have a high fever for more than four days, if the fever goes down but then starts to rise again, or if you belong to one of the flu vaccination groups.
People who are pregnant, aged over 65 or have certain chronic conditions are advised to get a flu vaccine, which is offered everywhere in Sweden although the cost can vary by region.
Symptoms include a fever, headache and cough.
Normal and mostly harmless flu symptoms are not dissimilar from the coronavirus, which has killed more than 1,000 people – almost all in mainland China – since it broke out in the city of Wuhan in late December.
But it is important to be aware that the Swedish Public Health Agency deems the risk of the coronavirus spreading in Sweden to be very low. So far, only one person has tested positive for the virus, and she is reported to have minimized her social contact which means a low risk of infecting others.