Internet sensation Lil Bub -- a kitten-size cat with 2.3 million Instagram followers, an album, a book, a stage production and a talk show -- died Sunday, according to her owners, Mike and Stacy Bridavsky, who say Lil Bub "unexpectedly passed away peacefully in her sleep."

Lil Bub had been battling health problems ... including what her humans say was a persistent and aggressive bone infection. Still, they weren't expecting her to die so suddenly and without warning.

LB was a major influencer with people and pets. The cat with an extra toe on each of her paws and osteopetrosis started the first national fund for special needs pets, and was the subject of groundbreaking biological and genetic research, helping raise over $700,000 for animals in need.

Not that she did it for fame, but Lil Bub did rub elbows with human celebs from time to time -- such as Robert DeNiro at the 2013 Tribeca Film Festival.

She even had a famous bloodline ... scientists say she was related to Ernest Hemingway's six-toed cats.

Fun-ish feline fact: The first 2 cat years are roughly equivalent to 25 human years, after which each year equates to 4 human years. So, by our standards, Lil Bub lived to become a middle-aged woman of 49.

RIP