Some bacteria enclose themselves inside capsules formed from polymers of sugar molecules called polysaccharides. The capsule acts a little like an outer coat. Encapsulated bacteria can be more difficult for your immune system to kill, and some species of encapsulated bacteria are responsible for a variety of common and often dangerous illnesses.
White blood cells called phagocytes engulf invaders then destroy them -- a little like "eating" then digesting the pathogens. This process is called phagocytosis. Macrophages, neutrophils and dendritic cells are the most important phagocytes in your immune system. They recognize invaders with the aid of molecules called toll-like receptors, which bind to a variety of molecules common in microbes but absent from human cells. Encapsulated bacteria are more difficult for phagocytes to recognize and better-equipped to survive even after being engulfed by a phagocyte.
Streptococcus is a genus of bacteria that includes some important human pathogens, principally S. pyogenes and S. pneumoniae. Both of these pathogens are encapsulated bacteria. The former is responsible for diseases like strep throat, impetigo, cellulitis and necrotizing fasciitis; the media often refers to necrotizing fasciitis as "flesh-eating disease" or "flesh-eating bacteria." As its name implies, S. pneumoniae is the most common cause of pneumonia. Their polysaccharide capsules help make both these pathogens more virulent and more dangerous to human health.
S. aureus bacteria are found in grape-like clusters when studied under the microscope. They typically colonize the nasal passages in humans. Like the streptococci, they are encapsulated bacteria, although their capsule is more properly called a "microcapsule" because you can only see it with electron microscopy. Doctors aren't entirely sure what role the S. aureus capsule plays in its virulence, but they do know that S. aureus can cause a variety of diseases like boils, meningitis (infection of membranes covering the brain and spinal cord), pneumonia, urinary tract infections, mastitis (inflammation of the breast) and toxic shock syndrome. These bacteria are especially problematic in hospital settings, where they can infect wounds in patients recovering from surgery. Strains collectively called methicillin-resistant s. aureus or MRSA are resistant to many common antibiotics and pose an increasing problem.
H. influenzae bacteria are found in both encapsulated and non-encapsulated strains. Seventy-five percent of healthy children and adults carry these bacteria in their nasopharynx, where the nasal cavities connect to the throat. In children under age 5, H. influenzae can cause acute meningitis; it's also responsible for some ear infections, respiratory tract infections, and cases of sinusitis. Type b encapsulated H. influenzae are typically the ones responsible for disease, since their capsule helps them ward off phagocytes. Vaccinating children with the polysaccharide found in the capsule protects them from contracting diseases caused by H. influenzae type b.