germ carrier
Medical term for a person who harbors and can transmit pathogenic microorganisms, often without showing symptoms.
A germ carrier can infect others even without showing symptoms themselves.
保菌者 is a more common synonym also meaning 'carrier' (of bacteria), while 病菌保有者 explicitly includes 'pathogenic bacteria' in the term.
Compound of 病菌 (pathogenic bacteria) + 保有者 (holder, carrier).