**Question:**Which of the following is NOT a disease that is horizontally transmitted by a vector?- Syphilis- Malaria- Rocky Mountain Spotted Fever- West Nile Virus**Explanation:**This question asks about diseases and their modes of transmission. A vector-transmitted disease is one that is spread by organisms, usually insects. Consider which of these diseases do not fit this category.

Introduction A disease is a sickness or illness characterized by specific symptoms and signs. A disease that results from an infection spread through blood-feeding anthropods (such as fleas, mosquitoes, and ticks) to humans and other animals is called vector-borne disease A vector is an organism that does not cause disease itself, while it spread infection from one host to another through the transmission of pathogens. Humans have been known to be the accidental host for vectors in many diseases. For example, relapsing fever. Some vectors have mammalian reservoirs for their zoonosis, but they may accidentally get into the human body. This suggests that the microbial agents function well in the mammalian hosts but as a result of some kind of exposure or accidental contact, the vector may attack the human and transmit the infectious microbial agent inside the human body where they can have parasitic effects. Thus, many vectors can have humans as accidental hosts.Explanations A. Syphilis:- It is a sexually transmitted disease (STD), person-to-person by direct contact with a syphilitic sore called a chancre, therefore this option is correct. B. Malaria:- Anopheles Mosquitoes are a vector of malaria which transmits it, therefore this option is incorrect. C. Rocky Mountain Spotted Fever:- Brown dog ticks (Rhipicephalus sanguineus) is a vector of Rocky Mountain Spotted Fever which transmits it, therefore this option is incorrect. D. West Nile Virus:- Mosquitoes of the genus Culex is a vectors of West Nile Virus which transmits it, therefore this option is incorrect.Conclusion Thus from the above explanations, we can conclude that Option A is the correct answer:- Syphilis.