In Denmark, an outbreak of Hepatitis A has so far infected 30 people. Frozen berries, especially strawberries, are believed to be the source of the infection.
Even Norway and Finland has seen an increase in the number of cases, writes the Swedish Institute for Communicable Disease Control (SMI) on its website.
Sweden, since the beginning of December, has reported 22 cases of infections from different parts of the country.
The normal figure is usually around five cases. A handful of those cases have been infected by the same type of hepatitis A as in Denmark.
SMI is now warning all Swedes to boil their frozen berries for at least a minute, to be on the safe side.
"Boiling applies to all types of frozen berries from all suppliers," says Margareta Löfdahl, epidemiologist at SMI.
"The infection spreads through faeces and in Denmark it is believed that it has spread through irrigation of berries but it is still only an educated guess," says Margareta Löfdahl.
SMI and the Swedish National Food Agency are now working together to investigate the cause of the investigation.
Samples of frozen berries have been sent for analysis.
The Hepatitis A virus is excreted via feces. It can be transmitted through food or through contaminated water. When food and water are heated the virus is killed.
Hepatitis A can be transmitted between people, but only in close contact, such as sexual contact.