What is an infectious disease characterized by high fever and skin discoloration and that historically has caused extensive mortality?

The infectious disease characterized by high fever and skin discoloration, historically causing extensive mortality, is smallpox. This serious disease, caused by the variola virus, was known for its distinctive rash and high fatality rate. Smallpox led to severe skin lesions, often leaving pockmarks on survivors and was a significant cause of death before vaccination efforts eradicated it in the late 20th century.

While the plague also caused high mortality and fever, it is primarily transmitted through fleas and is marked by different symptoms, such as buboes, rather than skin discoloration. Rabies is a viral disease that affects the central nervous system and presents with neurological symptoms rather than fever and skin changes. Tetanus, caused by a bacterial toxin, does not typically present with fever or skin discoloration but rather with severe muscle rigidity and spasms. Therefore, smallpox is the most fitting answer when considering the specific characteristics described in the question.