Theileria is a genus of protozoan parasites that primarily infects red blood cells in various animals, particularly cattle and sheep. These parasites are transmitted through the bites of infected ticks, leading to diseases such as theileriosis, which can cause severe health issues in livestock. The life cycle of Theileria involves both a vertebrate host and an invertebrate vector. Once inside the host, the parasites multiply within the red blood cells, leading to symptoms like fever, anemia, and in some cases, death. Effective control measures include tick management and vaccination to reduce the impact of these infections on animal health and agriculture.