When people move from one city or country to another, the spread of diseases may result. People often bring in germs which may not have been present there before. These new germs can spread quickly and cause previously unknown diseases. ______________ . They become ill more easily and could even die. In turn, newcomers may catch diseases which were not present where they came from.

The context presented in the given paragraph concerns how spread of diseases can follow or be aggravated by the moving of people from one city or country to another. The primary discussion is germs and how they can spread quickly. Since the sentence the follows the sentence in the blank mentions the word 'they', the sentence in the blank should talk about people. Only options C and D are good choices in this regard. Option A already uses the word 'they' and the transition from the pronoun use for germs versus for people is not grammatically sound. Option B mentions pollution, which has not been discussed in the paragraph, and also does not mention people. Option D talks about some positive changes that might enhance the lives of people, and is therefore out of context. The best choice out of the four options is option C.