After the rickettsia bacteria infects humans through a tick bite, it invades endothelial cells in the circulatory system (veins, arteries, capillaries).[13] The body then releases chemicals that cause inflammation, resulting in the characteristic symptoms like headache and fever. The hallmark of all rickettsial diseases is a histology (cellular) finding called lymphohistiocytic vasculitis[14] that involves immune cell deposition into the endothelial cells that make up vessels.[7] This occurs secondary to the chemicals mentioned above, as well as damage from the infection, and involves signals to immune cells (T cells and macrophages) to come to the site of the infection.[15]