Simplify the following text for a Grade 6 native English-speaking person.
Original: The population has been growing continuously since the end of the Black Death , around the year 1400 ; at the beginning of the 19th century , it had reached roughly 1,000,000,000 ( 1 billion ) .
Simplified: The population has been growing since the end of the Black Death , around the year 1400 . At the beginning of the 19th century , it had reached roughly 1,000,000,000 ( one billion ) .