This chapter discusses the reasons why early diagnosis and successful treatment of infectious diseases in the older adult is a challenge to the healthcare provider. Infectious diseases such as pneumonia with a high risk of mortality as well as other infectious diseases (COVID-19, influenza, urinary tract infections, sepsis/septic shock, tuberculosis) are discussed in relation to risk factors, assessment tools, antimicrobial therapy and potential drug-drug interactions and adverse drug reactions. Recommended vaccines and their efficacy in the older adult are presented. In an effort to prevent antimicrobial drug resistance, prescribing strategies from the Antimicrobial Stewardship are reviewed.