Written in question and answer format, this chapter aims to cover a brief discussion on infectious diseases. Crisp and to the point answers of the questions have been provided throughout this chapter. Pyrexia (fever) of unknown origin (PUO), bacterial infections, pneumococcal infections, dysentery, rickettsial diseases, viral infections, fungal infections, opportunistic mycoses, protozoal infections, cestodes (tapeworms), ectoparasites, sexually transmitted infections, chlamydial infections, granuloma inguinale (donovanosis), syphilis, nosocomial infections (hospital infections), sepsis, anti-infective therapy, and drug resistance the topics which are covered throughout this chapter.