Recessive genetic diseases are caused by inheriting two altered copies of the gene responsible for the pathology. In other words, when a child inherits the altered allele from both the father and the mother. In this case, the parents are healthy carriers of the disease.
Many people are not aware that they have an altered allele in their DNA and that they are carriers of a genetic disease, as they do not suffer from it. Only if they reproduce with another person with the same genetic situation, the couple will have a 25% risk of having a sick child with the disease.
On the other hand, parents who are carriers of a recessive genetic disease will also have a 50% chance of having a healthy carrier child like themselves. There is only a 25% chance of having a healthy child without any altered copy of this gene.