Is it possible for a human to live forever if he/she didn't catch any diseases?
No that is not possible ever. There is something called telomere which is present in the chromosomes of cell .These Telomere triggers the action of ageging and leads body towards death. When we are in growing age the amount of telemere is very less but as we get older and older the amount of telemere get increases .
The rate of increase of telemere increases with each succesive cell disivision. Hence death is sure no matter what ever you do.