BERLIN — As mysterious cases of a rare and ominously named virus began surfacing in Europe, Germany’s disease-control center quickly told people to be on the lookout. In a May 19 alert, the agency listed telltale symptoms of monkeypox: fever, aches, a rash. Then, in a further comment that set different alarm bells ringing, the bulletin pointedly warned men who have sex with men to “seek immediate medical attention” if they detect signs of the disease. The singling out has sparked fears that gay and bisexual men, who appear to account for the majority of Europe’s monkeypox cases so far, are once again in danger of being stigmatized as carriers of an exotic and frightening disease, just as they were during the AIDS crisis.