Chlamydia can not survive without humans, because it needs ATP, an energy molecule, to reproduce -- as well as other nutrients and supplies that it can't make it on its own. Basically, chlamydia treats the insides of human cells as great big grocery stores, and, since the bacteria can't live without groceries, C. trachomatis is as an obligate (can't survive without) intracellular (living inside cells) parasite (where it takes but does not give back).
Chlamydia travels between cells, and between people, in the form of an elementary body, which is a small, dense, spore-like, metabolically-inactive structure, almost like a virus, that doesn't do much of anything. It's infectious but inactive. When it enters a host cell, it changes into a reticulate body and uses supplies from the host cell to make copies of itself inside the cell. These reticulate bodies can grow, divide, and metabolize, but they're not infectious. Infections can persist in this manner for a while, or, once there are enough copies, the reticulate bodies can turn back into elementary bodies, burst the cell open, and escape to infect new cells or new people.
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