Mitochondria are small organelles found in nearly every cell in the human body. And they perform a function essential to human life, converting glucose from food to energy called adenosine triphosphate, or ATP, which cells can use.
But mitochondria carry their own genetic material -- separate from human DNA -- and these genes look a lot like those of bacteria. In other words, it's very likely that the mitochondria that we depend on for survival are the products of an ancient infection [source: Andersson et al.].
Whatever the infection, it predates animal life, let alone humans. So there's no use exploring the fossil record. Instead, researchers compared the genes of mitochondria to those of existing bacteria. The closest match was to bacteria of order Rickettsiales, many of which cause diseases -- including Rocky Mountain spotted fever [sources: Eremeeva and Dasch, Andersson et al.].
But remember, we're talking about a disease that existed before animal life. So the oldest disease isn't really Rocky Mountain Spotted Fever itself, but some unnamed proto-disease with genetic similarity.
Long, long ago bacteria invaded a cell. And because of this infection, we have life as we know it.
Read on to the next page for more infectious articles.
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HowStuffWorks looks at the first results of a scientific experiment that will run for 500 years.