Bacteria

A bacterium is a unicellular microorganism.

• There are around five nonillion (5 × 1030) bacteria in the world. (Wikipedia)
• There are 10 times more bacterial cells than human cells in the human body.

Viruses

A virus is a microscopic particle that can infect the cells of a biological organism. Viruses can only replicate themselves by infecting a host cell and therefore cannot reproduce on their own.

Are viruses living organisms? It has been argued extensively whether viruses are living organisms. Most virologists consider them non-living, as they do not meet all the criteria of the generally accepted definition of life. They are similar to obligate intracellular parasites as they lack the means for self-reproduction outside a host cell, but unlike parasites, viruses are generally not considered to be true living organisms. A definitive answer is still elusive because some organisms considered to be living exhibit characteristics of both living and non-living particles, as viruses do.

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