Simplify the following text for a Grade 10 native English-speaking person.
Original: A significant human pathogenic bacterium , S. pneumoniae was recognized as a major cause of pneumonia in the late 19th century , and is the subject of many humoral immunity studies .
Simplified: An important human pathogen , S. pneumoniae was recognized as a major cause of pneumonia in the late 19th century and is the subject of many humoral immunity studies .