Look at how Alexander Fleming discovered penicillin. Upon returning from a vacation, Fleming checked on colonies of staphylococcus bacteria growing in Petri dishes in his laboratory. Bar one dish that had been kept uncovered, next to an open window. He found that the bacteria surrounding a mould that grew in that dish had all died. The mould, a fungus called penicillium notatum, was isolated, cultured and produced in quantity, giving us penicillin, the first modern antibiotic.
