Comedian and actress Rosie O'Donnell, 50, announced on Friday that her fiancée Michelle Rounds has a serious illness that caused the couple to postpone their wedding until next summer.

The couple publicly announced their plans to marry in December, but recently found that Rounds suffers from what O'Donnell describes as a "beyond rare" disease.

Rounds began to experience pain and the couple did not know what was wrong with her, until she was ultimately diagnosed with desmoid tumors. She is now recovering from surgery she had in June, O'Donnell wrote on her blog.

So what exactly are desmoid tumors? According to the Desmoid Tumor Research Foundation, they arise from connective tissue, which are the cells involved with the formation of muscle, fibrous and nerve tissue. They can grow into and even destroy adjacent normal tissues, even bones. Desmoid tumors are not able to spread distantly throughout the body, however.

While the disease is very rare,individuals between the ages of 15 and 60 are most often affected, the site notes. Though they are not able to spread throughout the body, desmoid tumors are locally aggressive, sometimes causing destruction of adjacent vital structures and organs.

O'Donnell in a poem on her blog stated: "Only 900 cases a year in the U.S./An Orphan disease --beyond rare/ Only 3 million people per million get this/ Michelle is one of them." The disease isn't cancerous but "acts cancerous."

"Life changes in an instant," O'Donell added.

Desmoid tumors are considered benign, according to the Desmoid Tumor Research Foundation, but that doesn't mean they aren't dangerous because they grow quickly just like cancer.

O'Donell said that her fiancé is getting stronger every day, and they are now hoping to raise awareness about this disease, and instead of wedding gifts, they are asking for donations to the Desmoid Tumor Research Foundation.