Twelve cases of the Zika virus have been registered with German authorities since it became compulsory to do so on May 1, local media reported Wednesday.
"We assume that all those infected contracted the virus while on a trip abroad," a spokeswoman for the Robert Koch Institute, a governmental public health organization responsible for disease control, told Neue Osnabruecker Zeitung.
The cases registered in May take the total number of people who have contracted the virus and alerted authorities since October to 56.
"There is likely a large number of unknown cases - the virus usually takes a very mild course and patients don't go to see a doctor," the report cited the spokeswoman as saying.
The mosquito-borne virus, which can also be transferred sexually, has spread across more than 60 countries and territories in the Americas since an outbreak was detected in Brazil last May.
While most Zika patients recover quickly, the virus can cause brain malformations in unborn children, as well as other neurological disorders.
Health agencies have recommended pregnant women avoid mosquito bites and consult doctors before planning a trip to countries where the virus is circulating.
On Tuesday, the World Health Organization (WHO) warned people who return from regions affected by the virus not to engage in unprotected sex for eight weeks.