- Gram ⊕, lancet-shaped diplococci. Encapsulated. IgA protease. - Optochin sensitive.- Quellung-reaction-positive (capsule swelling on microscopy after exposure to capsular antigens)

Most common cause of:- Meningitis- Otitis media (in children)- Pneumonia- Sinusitis

- Pneumococcus is associated with "rusty" sputum, sepsis in patients with sickle cell disease, and asplenic patients.- No virulence without capsule.