The emergency clinic at the hospital has been closed and staff who have been in contact with the patient are being looked after.
The patient was admitted to a hospital in Enkoping before being transferred to nearby Uppsala University Hospital, where the emergency clinic has been closed.
Staff who have been in contact with the patient - whose recent travel has not been revealed - at both hospitals are also being looked after, the regional authority said.
Test results are expected on Friday night and officials have stressed that the disease may not be Ebola, and that for now it is "only a matter of suspicion".
Of 585 people reported to have been infected there in the last six months, 356 have died.
The epidemic is only surpassed by the outbreak that crippled western Africa between 2013 and 2016, during which more than 28,000 cases were confirmed.