Giemsa stain is a staining reagent that was developed in the early 20th century by Gustav Giemsa to aid in cellular microscopy. Different cells and parts of cells stain slightly different colors, allowing appropriate diagnoses to be made.
Giemsa stain is frequently used to diagnose parasite infections as the parasites stain differently than the surrounding cellular tissue. For example, cells infected with Chlamydia have lighter inclusion bodies visible after staining with Giemsa stain.
The stain is also used to visualize blood samples as the different blood cells stain different colors allowing for quick visual differentiation under the microscope. The stain is necessary because unstained cells are nearly transparent and illnesses are difficult to diagnose.