While this news may not be new to most, I have not seen any article on bloodyelbow on the matter.
Here's an article by Sherdog.com's Mike Whitman on the subject.
EDIT: I'm not saying that Sherdog give enough information to back their claim in their article, but they do state that it was confirmed (see bold part of article).
Roy Nelson just did not look right on Saturday night.
While many MMA fans and pundits immediately pointed to the fighter’s conditioning and unconventional physique as the source of his problem, Nelson has sported a large midsection for years without displaying the type of fatigue seen on Saturday. To that end, Sherdog.com has confirmed that Nelson suffered from a case of walking pneumonia prior to his clash with Frank Mir at UFC 130.
Yahoo Sports first reported that Nelson had dealt with the condition for the first two weeks of May prior to his bout with Mir, citing text messages exchanged with Nelson’s wife. Sherdog.com confirmed the news with Nelson, who asserted that walking pneumonia was indeed the culprit behind his sub-par conditioning.
However, Nelson was quick to give Mir his due, telling Sherdog.com that "Frank [was] the better man" in their scrap at the MGM Grand Garden Arena in Las Vegas.
When asked if he had recovered from the illness, the former International Fight League heavyweight champion answered frankly.
"[I’m] not feeling better," Nelson said. "Feels like I picked a scab off of my lung."
On a positive note, Nelson expects to feel back to normal in "a couple of weeks" and is currently taking medication to recover from the sickness. "Big Country" also revealed that he plans on making some changes the way he goes about his business in the future.
"[I’m going to] live the dream to the fullest, make opportunities, and use them."