In short
Bacteria are single-celled organisms with no nucleus. They are found almost everywhere, and some cause disease while many are useful.
A bacterium is a prokaryote, usually to long. They are classed by shape: cocci (round), bacilli (rod-shaped), spirilla (spiral) and vibrios (comma-shaped). They multiply quickly by binary fission, and a cell can divide every minutes in good conditions. Useful bacteria make curd from milk, fix nitrogen in the soil and help us digest food. Others cause cholera, tuberculosis and typhoid, which antibiotics can treat.
In ideal conditions one bacterium divides every minutes. After hours it has divided times, giving bacteria.
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