New words：

bacteria [bækˈtɪriə] n. 细菌（复数）
bacterium [bækˈtɪriəm] n. 细菌（单数）

Sample Sentences：

All bacteria are larger than viruses.
The bacteria are harmless to humans.
Soap destroys the cell walls of bacteria.

virus [ˈvaɪrəs] n. 病毒

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