Four male patients at the public Queen Elizabeth Hospital in Yau Ma Tei are carrying a drug-resistant superbug that a microbiologist says is like "having a bomb with them".
The vancomycin-resistant enterococcus (VRE) bug can exist in the body without causing infection, but can kill if it infects the brain or enters an open wound.
A fifth patient being treated in the neurosurgery department died yesterday, but his death has been attributed to his underlying condition rather than the bug.
Experts say an outbreak of a superbug like VRE indicates a serious deficiency in a hospital's infection control regime and shows that "the environment is too dirty or congested".
Dr Ho Pak-leung, a microbiologist at the University of Hong Kong, said: "VRE usually has a very low attacking power and poses little threat to most people, but to these carriers who are also suffering from serious brain conditions, it is like have a bomb with them all the time."
He said it would cause "catastrophic conditions" if it entered a wound or the patient's brain.