How would I make a chance script? Like, a 0.05 chance that you will spawn with a disease.

Pretty much, what you have to do is make a math.random that arranges the amount of numbers that you need and then make an if condiction which will decide whether to return true or false. Per example, to make a 10% chance you would do something like this:

local chance = math.random(1, 10)

if chance == 1 then
print("won the 10% chance")
end

just remember that math.random can only give int values, therefore you will need to increase the number range as the percentage lowers