This chapter covers all the commonly tested infectious disease topics. ID, as such, is not a very hugely popular speciality in USMLE except for Pneumonia and HIV/AIDS. HIV/AIDS matches all other topics combined in ID and therefore extra attention has been paid to this topic. ID case encounters may be easy to diagnose, but need extra attention to the encounter, which is highly recommended, because small details help find the correct antibiotic. Other notable topics are Meningitis, Flu, Diarrhea, Pneumonia, Hepatitis, Pyelonephritis, Endocarditis. Few of the quick reference tool in this chapter are CSF analysis, Predisposing factors for specific pneumonia causative agent, Predisposing factors for specific diarrhea causative agent, Clinical features associated with specific diarrheal agents, Hepatitis viruses table, STDs table, Opportunistic infections table, HAART and adverse effects.