资料题：Insects and rats were dangerous because they,（　）。

For every man in the Civil War who died in battle, two or three men died of disease.Doctors of that time knew very little about causes .of sickness or ways of preventing it.Thousands of men in poor health became soldiers.Many of them could not resist epidemic (传染) diseases that went through the places where they lived.Amy life was hard.Soldiers got little fruit or vegetables.There was no milk unless they happened to find a cow.Neither their clothes nor their living places protected the troops from rains, snow and cold.Insects, rats, and unclean drinking water spread sickness and disease.Often the men drank straight form muddy streams.Gunshot wounds were serious, as in any war, but they did not cause as many deaths as disease did.