Cystic fibrosis is a deadly disease of the lungs and digestive system. it is inherited as a recessive trait. how can two healthy parents have a child with cystic fibrosis?

We'll make the alleles: Cc.

C is a healthy parent, and c is the cystic fibrosis (recessive).

If you make a punnett square and the parents are both Cc (healthy), then you would get CC, Cc, Cc, and cc. That cc is a 25% chance that the offspring will have that, which means there is a 25% chance the child will have cystic fibrosis.

Two healthy parents can have a child with cystic fibrosis if both the parents have the dominant allele and recessive allele, if both the parents have heterozygous alleles.