A heart murmur is a common diagnosis in children, but a heart murmur alone is not an illness or a disorder, reports KidsHealth. This type of heart murmur is called an innocent murmur, and it occurs when blood flows faster than normally through the heart, notes Mayo Clinic.
Innocent heart murmurs can be caused by anemia, fever, exercise and phases of fast growth, states Mayo Clinic. An abnormal heart murmur is serious and is caused in children when they are born with congenital heart defects. This includes heart valve irregularity and holes in the heart. Older children may suffer abnormal heart murmurs due to rheumatic fever, valve calcification and infections, such as endocarditis.