Is this a sentence fragment?
In the early 1800s, yellow fever, a viral disease spread by mosquitoes, not by unsanitary conditions as was believed at the time.
A: no
B: yes
This is a sentence fragment. It does not express a complete thought.
In the early 1800 s, yellow fever, a viral disease spread by mosquitoes, not by unsanitary conditions as was believed at the time.
Here is one way to fix the sentence fragment:
In the early 1800 s, yellow fever, a viral disease, was spread by mosquitoes, not by unsanitary conditions as was believed at the time.