The patient in this question has pneumococcal pneumonia, which must be considered in any patient with chills, fever, chest pain, and cough productive of purulent, rust-colored sputum. Streptococcus pneumoniae is an alpha;-hemolytic, gram-positive coccus that grows in chains. It can be easily distinguished from other alpha;-hemolytic streptococci because it is exquisitely sensitive to bile and bile like compounds, such as optochin. It is the most common cause of community-acquired pneumonia and the most common cause of community-acquired meningitis in adults older than 30. The only recognized virulence factor of S. pneumoniae is its carbohydrate capsule (which contains acidic polysaccharides). Antibody to a specific capsule type is necessary to overcome infection. More than 80 capsule types have been recognized. The 23 types that most commonly cause disease are contained in a vaccine that is recommended for high-risk groups, including the elderly and those undergoing splenectomy. Increased susceptibility is also found in patients with Hodgkin disease, chronic lymphocytic leukemia, and myeloma. The vaccine should still be given to patients with these conditions, but it is less successful. The alternative complement pathway is impoant in clearing Neisseria infections. Individuals with deficiencies in C5 through C8 are at increased risk of disease from Neisseria. The fimbria of Streptococcus pyogenes (Group A alpha;-hemolytic streptococcus) is composed of an a-helically coiled M protein. Antibody against a specific M type will prevent infection. However, raising antibodies to M proteins can lead to rheumatic fever, so Strep throat infections are routinely treated with penicillin to prevent an antibody response. The C carbohydrate is an antigen of alpha;-hemolytic streptococci used to divide them into different groups. Antibody against C carbohydrate is not protective. Ref: Brooks G.F. (2013). Chapter 14. The Streptococci, Enterococci, and Related Genera. In G.F. Brooks (Ed), Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.