The Ankaran flu was a contagious disease that required several days of treatment.
In 2375, The Doctor told "DaiMon Torrot", one of the personalities surfacing in Seven of Nine, that he had been treating "him" for Ankaran flu and that "he" needed to return to sickbay. (VOY: "Infinite Regress")
The Doctor claimed that one of the symptoms was short-term memory loss, but he was likely only trying to get "Torrot" to cooperate.
In 2376, while talking about Chakotay's boxing days, B'Elanna Torres jokingly warned Harry Kim that "the Tattooed Terror has put more men in sickbay than the Ankaran flu." (VOY: "Tsunkatse")