This is part of the Public Health (Control of Disease) Act 1984, sections 33 and 34: Public Conveyances. The taxi driver has a right to refuse anyone who may have a notifiable disease, or they may elect to transport you and then immediately disinfect their vehicle (unlike a bus driver, who must refuse you altogether). Some of the other notifiable diseases listed include plague, rabies, food poisoning and leprosy.
There are lots of references to this law, but it may be directed toward the drivers or passengers of motorcycles.
Sec. 7-33. Traffic. (a) State motor vehicle laws apply. No person in any city cemetery shall fail to comply with all applicable provisions of the state motor vehicle traffic laws in regard to equipment and operation of vehicles together with such regulations as are contained in this and other ordinances. (b) Enforcement of traffic regulations….