The B19 virus, also known as parvovirus B19, is a small virus that primarily causes a mild illness called fifth disease, or erythema infectiosum. It is most common in children and is characterized by a distinctive rash on the face, often referred to as "slapped cheek" rash. The virus spreads through respiratory secretions and can also be transmitted from mother to fetus during pregnancy. In addition to causing fifth disease, the B19 virus can lead to more severe complications in individuals with weakened immune systems or certain blood disorders, such as sickle cell disease. Most people recover without treatment, but those at higher risk may require medical attention to manage symptoms.