Answers

2016-01-23T13:57:43+08:00
Dirty, small, disease, harmful, present in some food, used in making medicine, vaccine.
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2016-01-23T14:13:55+08:00

The first thing that comes to my mind is disease because a lot of them are disease producing, second is decomposition of waste because there are also plenty of these organism involved in decomposition of biodegradable materials
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