Simplify the following text for a Grade 9 native English-speaking person.
Original: Chlamydia infection is one of the most common sexually transmitted infections worldwide ; it is estimated that about 1 million individuals in the United States are infected with chlamydia .
Simplified: The infection is one of the most common sexually transmitted infections in people worldwide -- about 2.8 million cases of chlamydia infection occur in the United States each year .