SEOUL—Missteps contributed to the spread of the deadly virus known as MERS at the prestigious hospital where South Korea’s richest man is receiving treatment.

Samsung Medical Center is the epicenter of the country’s fight against Middle East respiratory syndrome. In May, an infected man spent two nights in the emergency room there without being diagnosed, and health officials say he transmitted the virus to at least 80 people in the hospital before being quarantined.

“During that time, it is clear this person was coughing a lot and was also mobile in the emergency room,” Keiji Fukuda, assistant director-general for health security at the World Health Organization, said at a briefing on Wednesday.

South Korea’s MERS outbreak is in its fifth week. Officials on Thursday said four more people nationwide had died, taking the total number of deaths to 23. There were also three new MERS cases, taking the total to 165. Most patients who died were elderly and had other serious illnesses.

The 1,900-bed Samsung Medical Center has closed its doors to most of the public, though it continues to treat and receive MERS patients. Officials say there are also no plans to move Samsung Group Chairman Lee Kun-hee. The 73-year-old has been receiving care in a VIP suite near the top of the hospital since last year following a heart attack.