In the Age of Discovery, Europeans were playing with fire. Every voyage of exploration risked bring back some new plague. From the New World, syphilis, probably typhus and rheumatoid arthritis. From India, cholera. HIV, recently, from Africa. Comparably important new pests attacking important crops and domesticated animals also arrived, such as grape phylloxera (which wiped out most of the vineyards of Europe) and potato blight ( an oomycete or ‘water mold’, from central Mexico).
If one of those plagues had been as potent as smallpox or falciparum malaria, you probably wouldn’t be reading this.