The speculation can come to an end. According to Sherdog, Lesnar’s infamous illness is a bacterial infection of his intestinal tract. From the story:
Brock Lesnar is being treated in an unidentified hospital for a bacterial infection in his intestinal tract, UFC President Dana White told the Associated Press on Monday.
“He’s in stable condition and should be released soon,” said White.
…
When asked if Lesnar would fight again, White told the Associated press that, “It depends on how serious this is.”
Cursory research provides the following. Bacterial gastroenteritis generally improves with replacement of fluids within a week, though “rare cases of kidney failure or death” have been reported. The infection can be caused by E. Coli, but Medicenet.com states “Enterocolitis caused by the bacterium Clostridium difficile is unusual because it often is caused by antibiotic treatment…Unfortunately, infection also is increasing among individuals who have neither taken antibiotics or been in the hospital.”
Antibiotic’s are not used to treat mono on its own, but if Lesnar had another illness like strep throat, he would have been prescribed them.
[UPDATE] by Nick Thomas – twitter.com/DanaWhite