CF can't be caught or developed. If you are not born with CF you will never have it, but you could be a CF gene carrier. However, some people with CF are not diagnosed until later in life.
One in 25 people carry the CF gene. For someone to be born with CF, both parents must carry the faulty gene. If both parents have the gene, there is a 25% chance the child will have CF. If both parents carry the gene there is also a 50% chance of the child being a gene carrier but not having CF and a 25% chance they will not have the CF gene.