If the presidential nominee fell ill a month before the final vote, what happens?

I’m wondering for American elections, but you can answer for other countries. Let us know what country.

Let’s say the illness is a type that the person can’t be president.

Whether it be an incumbent president (that would be Trump this year) or the nominee who just decided by a party (that’s going to be Biden or Bernie this year I guess).

Would Pence be one the automatic person on the Republican ballot?

Would the VP pick by nominee become the nominee, or would it suddenly be the person who had the second most delegates?

I hope I worded that so it’s understandable. Thanks!