Hugh Hefner and wife Crystal Harris. Picture: Bang Showbiz

Hugh Hefner was struck down with two years of serious back issues and then caught a mild bug shortly before he died which he was unable to fight.

The Playboy founder - who passed away from "natural causes" on Wednesday aged 91 - caught a mild bug recently but he was unable to fight it because his immune system was so weak, according to gossip website TMZ.

He had back problems in his 80s but these are said to have become a lot worse two years ago when he was struck down with an infection, and he was reportedly left bedridden towards the end of his life. 

One of the businessman's final public appearances came in May 2016 when he announced Eugena Washington was 2016's Playmate of the Year.

Read: Hugh Hefner to be buried next to Marilyn Monroe

Several of Hugh's loved ones, including daughter Christie, 64, and sons Marston, 27, and Cooper, 26, were said to be at his bedside when he passed away. 

His second wife Kimberley Conrad - who he was married to from 1989 to 2010 - was also thought to be in attendance. 

Hugh - who had son David, 62, with first wife Millie Williams - died in the famous Playboy Mansion, where he used to hold wild parties. 

The magazine founder - who left behind wife Crystal Harris, 31 - is set to be buried next to Marilyn Monroe after purchasing the crypt next to the Hollywood icon's at the Westwood Village Memorial Park in Los Angeles for $75,000 in 1992. 

Hugh previously revealed his pal Jay Leno suggested he buy the plot.

He said: "Jay Leno suggested that if I was going to spend that kind of money, I should actually be on top of her.

"But to me there's something rather poetic in the fact that we'll be buried in the same place. And that cemetery also has other meanings and connections for me. Friends like Buddy Rich and Mel Torme are buried there. So is Dorothy Stratten."

Marilyn was the first-ever cover star in Playboy magazine, which was first published in 1953 and the issue sold over 50,000 copies.