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.

A

If they had gone back, they would have started an epidemic.

B

Pollution also can lead to the spread of disease.

C

If a germ is completely new to a region, people have no natural protection against it.

D

Such changes may result in enhancing conditions for people who live in big cities.