According to the KCDC, those infected are master's and doctoral candidates who used the school's lab from Oct. 19 to 28. They are being treated in isolation at state-run hospitals.

Health authorities have placed all 21 in quarantine and closed off the school building. The Korea Centers for Disease Control and Prevention on Wednesday said the victims started coming down with pneumonia last week, and a team of epidemiologists have been dispatched to the university.

The first four graduate students who showed symptoms were hospitalized at Konkuk University Medical Center and then transferred to the National Medical Center on Wednesday. All of them visited a cattle fair in Gyeonggi Province last week as well as an animal farm owned by the university in Chungju, North Chungcheong Province.

A university official said they may have contracted Brucellosis, which infects cattle and can be transmitted to humans.

But some experts suspect Q fever, caused by the Coxiella burnetii bacteria found in the droppings of cattle, swine and sheep. It causes airborne infections among humans. Symptoms such as fever, headache and muscle aches appear after a two-week gestation period.

But Song Dae-sup at Korea University said, "Brucellosis and Q fever are not commonly found in Korea and are rarely passed on to humans. We need to look at the possibility of pneumonia caused by germs or other toxic agents."

Lee Jae-gap at Hallym University Medical Center said since only the four visited the animal farm, the infection could be mycoplasma pneumonia or whooping cough, which sometimes occur in schools or communities.

Konkuk University sealed off the veterinary school building and told the school's 850 staff and students to report any suspicious symptoms. The building was thoroughly disinfected on Wednesday morning as well, according to school officials.