Cystic Fibrosis is a genetic disease that babies are born with.  Healthy people have 46 chromosomes.  23 are received from their mom and 23 are received from their dad.  A gene is basically the code that makes up the physical structure of a person's body.  This is why children often look similar to their parents. The functions of most genes are to instruct the cells to make specific proteins, most of which have important life sustaining roles.

Chromosome 7 is the gene that sustains the lungs, pancreas, and other organs that turn food into energy for the body to grow and be sustained. When the mom and dad both have a symptom-less but still partially mutated chromosome 7 gene and have a baby; that baby has a 25% chance of receiving a fully mutated chromosome 7 gene. The mutated gene doesn't handle salt properly. Instead, it creates a mucus that coats the cells in the lungs and pancreas. In the lungs, this causes airways to become blocked and increases the risk of the lungs becoming inflected by bacteria. In the pancreas, this mucus makes it so digestive enzymes can't get to the intestines. Without this, the intestines cannot properly digest food. This makes it so people with CF do not get the nutrition they need to properly grow.

Information from this article was obtained with permission from Cffutah's article, Think you know what Cystic Fibrosis is?