Although bacteria and viruses are both too small to be seen without a microscope, they're structurally as different as giraffes and goldfish.
Bacteria are relatively complex, single-celled creatures with a rigid wall and a thin, rubbery membrane surrounding the fluid or cytoplasm inside the cell. Shaped like balls, rods, or spirals, they contain all of the genetic information needed to make copies of themselves. Fossilized records show that bacteria have existed for about 3.5 billion years, and it's known that bacteria can survive in a variety of environments, including extreme heat and cold, radioactive waste, and the human body.
Most bacteria are harmless, and some -- such as the Lactobacilli acidophilus bacteria that can live in the human intestine -- actually help digest food, destroy disease-causing microbes, fight cancer cells, and provide essential nutrients. Fewer than 1% of bacteria cause disease in people.
In contrast, viruses are tiny: the largest of them are smaller than the smallest bacteria. Viruses come in varied shapes, and have a limited genetic blueprint. All they have is a protein coat and a core of genetic material: either RNA or DNA. Unlike bacteria, viruses can't survive without a host. They can only reproduce by attaching themselves to cells and hijacking the cells' cellular machinery. In most cases, they reprogram the cells to make new viruses until the cells burst and die. In other cases, they turn normal cells into malignant or cancerous cells.
Also unlike bacteria, most viruses do cause disease, and they're quite specific about the cells they attack. For example, certain viruses are programmed to attacks cells in the liver, respiratory system, or blood. In some cases, viruses called bacteriophages target bacteria.
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