Influenzal meningitis is rarely seen under two months of age or beyond six years of age. About 80 per cent of the cases occur between the ages of two months and three years.
The bactericidal power of whole defibrinated blood in various age groups for a meningitic strain of B. influenzae has been studied.
Cord blood and the blood of infants under two months of age has considerable bactericidal power.
With rare exceptions the blood of children between the ages of two months and three years has practically no bactericidal power.
After three years the bactericidal power gradually rises and appears to be permanent in adult life.