From the article:

"It [the Robert Koch Institute] said laboratory tests to determine the source of the infections were not expected to be available for several weeks. [...] The [Robert Koch] institute said malaria in Germany almost exclusively affects long-haul travellers who were infected in malaria-endemic areas, and that malaria transmitted within a German airport is rare. 'The lack of a travel history can lead to a delayed diagnosis. A delayed diagnosis increases the risk of a severe course of the diseases,' the institute said in its epidemiological bulletin."