Since Joe Biden took office, about two dozen U.S. intelligence officers, diplomats, and other government officials in Vienna have reported experiencing mysterious afflictions similar to the Havana Syndrome. U.S. officials say the number of possible new cases in the Austrian capital—long a nexus of U.S. and Russian espionage—is now greater than the number reported by officials in any city except for Havana itself, where the first cases were reported. The exact cause of the ailments in Vienna, which U.S. government agencies formally refer to as “anomalous health incidents” or “unexplained health incidents,” remains unknown, but in response to the surge the C.I.A., the State Department, and other agencies are redoubling their efforts to determine the cause, and to identify the culprit or culprits. A C.I.A. spokesperson said that the agency’s director, William Burns, was “personally engaged with personnel affected by anomalous health incidents and is highly committed to their care and to determining the cause of these incidents.” Privately, Burns has called the maladies “attacks” rather than incidents. A State Department spokesman said, “In coordination with our partners across the U.S. government, we are vigorously investigating reports of possible unexplained health incidents among the U.S. Embassy Vienna community.” The Havana Syndrome derives its name from the Cuban capital, where C.I.A. officers and State Department employees first reported experiencing strange sensations of sound and pressure in their heads in 2016 and 2017.