Many of the alpha-hemolytic streptococci are part of the normal body flora. Many of them can be pathogenic when introduced into other parts of the body.

Viridans group streptococci cause dental caries as well as subacute bacterial endocarditis following dental surgery or injury to the oral mucosa. The viridans group include the alpha-hemolytic streptococci that lack Lancefield group antigens and do not meet the criteria for Streptococcus pneumoniae.

Streptococcus pneumoniae causes 60-70% of all pneumonias. It can be a very serious pathogen. Young children, the elderly, and immunocompromised individuals are particularly susceptible. It can also cause sinusitis, otitis media, bacteremia, and meningitis. Gram stain often shows leukocytes and numerous lancet shaped gram positive cocci in chains.