Both strains quickly invaded local hospitals when needle sharing and other unhealthy practices ensured the rapid spreading of the infection by bringing people into contact with contaminated body fluids.
It begins in the United Kingdom where health officials say Omicron, a newly discovered variant or version of COVID-19 is spreading rapidly throughout the country.
So in metropolises – the largest and most important cities, where people live crowded close together, diseases spread like wildfire – an idiom meaning spread widely and quickly.