Sources with knowledge of the matter told StreetInsider that "regular flu" was likely the cause of the fever that triggered isolation of a guest onboard Costa Smeralda Wednesday night.

The illness forced as many as 7,000 to be held on the cruise ship in Italy while authorities investigated the possibility the illness was from the new coronavirus, a disease that has already taken 171 lives and stricken 8,100 others. Coronavirus was later ruled out as a possibility.

Costa Smeralda is a Costa Cruises ship and part of Carnival Corp. (NYSE: CCL). It is operating a seven-day cruise in the western Mediterranean, including Civitavecchia, La Spezia, Savona, Marseille, Barcelona and Palma de Maiorca.