Simplify the following text for a Grade 9 native English-speaking person.
Original: Anemia ( , also spelled anemia and anÃ mia ; from Ancient Greek ' , meaning lack of blood ) is a decrease in number of red blood cells ( RBCs ) or less than the normal quantity of hemoglobin in the blood .
Simplified: Anemia ( U.S. spelling ) or anemia means not having enough red blood cells or hemoglobin in the blood .