How can a child inherit a disease from their parents if neither parent has the disease?

We inherit two copies of a gene called alleles, one from each parent. The alleles we inherit from our parents can either be dominant or recessive. A dominant allele requires just one copy of the allele to be expressed, whereas a recessive allele requires two copies of the recessive allele to be present, that is the dominant allele must be absent. In the question, a child is expressing alleles that are not expressed in the parents. It is because the parents have one dominant allele, the normal allele, and one recessive allele, the allele that causes the disease. The child inherited the recessive allele from both mom and dad and has therefore ended up with two recessive allele and so the disease.