MMA News reports:

A source close to the situation spoke to MMANews.com's Michael Moody on a condition of anonymity and explained that the blood tests came back right before Aleks and Buentello were scheduled to weigh in and that is why there was so much confusion around the stage and why the fighters did not weigh in. Gary Goodridge ended up stepping up on extremely short notice and filling in for the Russian heavyweight.

The tests were positive for Hepatitis B which resulted in the CSAC cancelling the fight. ...

A positive Hepatitis B test is no laughing matter and this could spell the end of Aleks' fighting career as many of the people close to the situation pointed out. I can't imagine many fighters willing to step up to fight Aleks with the possibility of catching the disease. He most definitely will not fight in the United States again and it will be interesting to see if this positive test results in organizations from all over the world black listing the brother of heavyweigh king, Fedor Emelianenko.

Some are concerned that the condition will end Aleksander's career but from a quick review of wikipedia, it appears that not all cases are permanent:

Acute hepatitis B infection does not usually require treatment because most adults clear the infection spontaneously.^[22] Early antiviral treatment may only be required in fewer than 1% of patients, whose infection takes a very aggressive course ("fulminant hepatitis") or who are immunocompromised. On the other hand, treatment of chronic infection may be necessary to reduce the risk of cirrhosis and liver cancer. Chronically infected individuals with persistently elevated serum alanine aminotransferase, a marker of liver damage, and HBV DNA levels are candidates for therapy.^[23]

Although none of the available drugs can clear the infection, they can stop the virus from replicating, and prevent liver damage such as cirrhosis and liver cancer. Treatments include antiviral drugs such as lamivudine, adefovir, tenofovir and entecavir, and immune system modulators such as interferon alpha. However, some individuals are much more likely to respond than others and this might be because of the genotype of the infecting virus or the patient's heredity. The treatment works by reducing the viral load, (the amount of virus particles as measured in the blood), which in turn reduces viral replication in the liver.