Cortes landed at Veracruz the next year, in 1519.
https://upload.wikimedia.org/wikipedia/commons/5/56/Acuna-So...
Cortes landed at Veracruz the next year, in 1519.
https://upload.wikimedia.org/wikipedia/commons/5/56/Acuna-So...
The reason that diseases from the old world were so deadly, when they crossed the pond, is that they'd evolved to spread quite well even in humans with an evolved or acquired resistance and were suddenly in a completely unprepared population.
If they are close in biochemistry the dangers are extreme to catastrophic. If we actually found a crashed UFO with bodies we should drop a thermonuclear bomb on it immediately. Just ask the Native Americans.
If they are not biochemically close chances are we and our environment would be horribly toxic, freezing, or boiling hot to them and vice versa.
Aliens probably won’t be human, so their diseases infecting us is unlikely at first