Exposure

• Exposure to H. influenzae occurs early in life and humans rapidly acquire immunity, states Jacquelyn Black in her book "Microbiology: Principles and Explorations." Everyone over the age of 6 has antibodies to H. influenzae, according to Black. A child will probably not get sick from the bacteria so long as they stay in the child's nose and throat, according to the Centers for Disease Control and Prevention.
  
  

Risks

• The outlook for individuals exposed to H. influenzae depends on several factors, including whether the bacteria move beyond the nose and throat, what type of tissue the bacteria invade, and whether the individual receives vaccination or other treatment. Only a small percentage of H. influenzae carriers develop invasive disease, according to E-medicine. If, however, the bacteria invade the lungs or bloodstream, they can cause serious problems, including pneumonia, meningitis, infections of the blood, joints, bones, and covering of the heart, and epiglottitis (infection of the area of the throat that covers and protects the voice box and trachea during swallowing), notes Boston Children's Hospital.
  
  

Hib Vaccine

• Introduced in the United States in 1988, the Hib vaccine protects against the most virulent strain of H. influenzae, strain B. This strain once accounted for more than 95 percent of H. influenzae invasive disease, according to E-medicine. Before the Hib vaccine, about 20,000 children in the United States under 5 years of age got severe Hib disease each year, according to the CDC. H. influenzae now occurs in fewer than two in 100,000 U.S. children, according to Medline Plus.
  
  

Newborns

• The success of the Hib vaccine in protecting against H. influenzae invasive disease should not be allowed to diminish appreciation of the dangers the disease can pose. Neonatal H. influenzae disease carries a mortality rate of 55 percent, according to E-medicine. Thanks to antibodies fetuses get from their mothers while in the womb, newborns typically have immunity to the bacteria, but after birth, as these antibodies wane, infants become highly susceptible to acquiring H. influenzae invasive disease, notes E-medicine.
  
  

Perilous Complications

• Without treatment, meningitis caused by H. influenzae is almost always fatal, notes Black. Even with treatment, 5 percent of victims die, states E-medicine. Of those who recover, 30 to 50 percent have serious mental retardation, according to Black. Despite the Hib vaccine, H. influenzae still causes 5 percent to 10 percent of bacterial meningitis cases in adults, observes Medline Plus. Epiglottitis caused by H. influenzae can lead to acute obstruction of the respiratory tract, with a mortality rate of 5 percent to 10 percent, notes E-medicine.