There are many risks involved with nautical exploration. One of them is that you trying to get to uncharted islands which can be difficult in itself because theyre uncharted. You dont know what the conditions will be like there and how the weather will be. Another risk of nautical exploration is that the natives in the courties you are exploring could be aggressive and hostile to non-natives exploring their country. Another risk is the risk of disease. It is very likely that when a group of people arrive in a place where no one but natives has been before, disease will spread because there hasnt been any chance to become immune to these diseases. This is also just as true for the natives that live in uncharted places.
They are not immune to our diseases either and are therefore just as easily affected by them as the explorers. Also, upon arriving at what would eventually be called America, Cabeza de Vaca's ship encountered a hurricane which destroyed some of his boats.
