The most common and severe form of sickle cell disease is sickle cell anemia (aka Hb SS). It occurs when you inherit the sickle cell beta-globin mutation (S) from both parents.
But people with sickle beta0-thalassemia (Hb SB0) can experience equally severe disease.
Other common forms of sickle cell disease, such as sickle beta+-thalassemia (Hb SB+) or Hb SC disease, tend to be milder.