The number of Covid-19 infections at Chulalongkorn University has risen to 22, with 13 of them being security guards.
Taweesilp Visanuyothin, spokesman for the Centre for Covid-19 Situation Administration, said on Tuesday that the first case at the campus was detected on Jan 29.
He was a 56-year-old messenger who lived on the 12th floor of the 15-storey Chula Nivas employees' dormitory in the university grounds.
The man then spread Covid-19 to six other people who lived on the same floor. Some of them were close relatives and three worked as security guards at the university.
The disease was then transmitted to 10 other security guards, three of them living in the same dormitory, Dr Taweesilp said.
"The security guards shared the same workplace, training grounds and finger scanners where the disease was found, and also had meals together... This group of people also transmitted the disease to five relatives," Dr Taweesilp said.
The case cluster, and especially the virus traces found on finger scanners, emphasised the necessity of frequent hand washing, he said.
Most cases in the cluster were people of working age, 40-49. They suffered from coughing and had excessive phlegm, muscle pains and had breathing difficulties, the spokesman said.