<unk><unk><unk>  fever is caused by a bacterial organism of the Rickettsia genus known as Rickettsia ricketsii. The organism is a Gram-negative bacteria, ranging in sizes between 0.2 x 0.5 micrometers to 0.3 x 2.0 micrometers. Rickettsia ricketsii is non-spore forming, non- motile, and varies in shape. Early in it's life cycle, Rickettsia ricketsii survives within an invertebrate host. The invertebrate host may then transmit the organism through blood meals, open skin, and breakages in mucous barriers. 
The disease is most often transmitted through arthropod vectors, especially the hard tick- Ixodidae family. Rickettsia ricketsii requires an invertebrate vector and vertebrate host. Humans are considered an accidental host within the R. ricketsii life cycle. 
The organism has evolved a set of strategic mechanisms in order to evade the human immune system. Infection begins with an induced phagocytosis of the organism into an endothelial host cell. Usually endothelial cells are not phagocytic, however R. ricketsii is able to induce changes to the overall cytoskeleton of the cell. Invasion goes largely unnoticed, enabling the organism to avoid lysosomal fusion and an escape into the cytoplasm for future reproduction.
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