When the bus arrived at the border, if there was someone ill, they would still have to report that there was someone ill to the customs officials, who could then, if they thought this was something that looked suspicious, call our quarantine officer in Vancouver, who would then take appropriate action.
Now, if people got ill after they entered the country, then we would still rely on the medical officers who were treating people to inform us, and then an epidemiological investigation and perhaps quarantine could be instituted, depending on what the disease was.