Answer:
Because
Step-by-step explanation:
So, basically, transition words are used to link two ideas expressed. They can follow the same idea explaining it in more detals (also, additionally, besides etc.), they can show contrast (conversely, contrary, on the other hand), they can show cause-effect link (since, because etc.), they can conclude something (so, finally, therefore etc.)...
Here, first sentence informs us on the epidemics and the second shows that it was the reason Knox fled to Manhattan. Since it shows why something happened, the best choice out of given options is BECAUSE.