The New York Post said Friday the bartender, whom it identified only as Leif, was diagnosed with Hepatitis A and is now being treated for the viral infection that affects the liver.

However, Socialista, the club where Demi Moore, Madonna, Lucy Liu, Salma Hayek, Kate Hudson and Bruce Willis helped Kutcher celebrate his 30th birthday Feb. 7, has issued a warning to people who were at the club that night, the following night or Feb. 11 to get tested and vaccinated for Hepatitis A.

TMZ.com said bar owners estimate about 700 to 800 people may have been at Socialista on those three nights.

"Although no additional cases of illness have been identified, the New York City Department of Health is urging customers to get the vaccination as a precautionary measure. We are grateful for their efforts and we will continue to support them in every way possible," the club said in a statement issued to TMZ.com.

Socialista's proprietor Armin Armiri told the Post, "We're not closed down, we're just concerned for our customers."