The second doctor watched for contagious and infectious diseases. He looked especially for infections of the scalp and at the eyelids for symptoms of trachoma, a blinding disease. Since trachoma caused more than half of all medical detentions, this doctor was greatly feared.

—Immigrant Kids,
Russell Freedman

What is the central idea of this passage?

The second doctor did more work than the first doctor.
Trachoma is a blinding disease that caused immigrants to be turned away.
Infections of the scalp and eyelids are dangerous to others.
The second doctor watched for contagious and infectious diseases.