Inhalation exposes the upper and lower respiratory tracts of humans to a variety of airborne particles and vapors. Airborne transmission of pathogenic microorganisms to humans from the environment, animals, or other humans can result in disease. Inhalation is an important route of exposure as the lung is more susceptible to infection than the gastrointestinal tract. Ingested microorganisms must past through the acidic environment of the stomach before they can colonize tissue while inhaled microorganisms are deposited directly on the moist surfaces of the respiratory tract. Inhalation of microbial aerosols can elicit adverse human health effects including infection, allergic reaction, inflammation, and respiratory disease. Following inhalation, infectious viruses, bacteria, and fungi can establish in host cells of the respiratory tract. Some are translocated and infect the gastrointestinal tract and other tissues. This chapter discusses human viral, bacterial, and fungal diseases transmitted via aerosols. Viral diseases presented are influenza, severe acute respiratory syndrome (SARS), Norwalk-like viruses (NLVs) and hantavirus disease, measles, and varicella. Bacterial diseases are Legionnaires’ disease, tuberculosis, and nontubercule mycobacterial disease. Exposure to some Gram-negative and Gram-positive bacteria, endotoxin, and actinomycetes when dispersed through the air can result in disease following inhalation. Fungal diseases included are histoplasmosis, coccidiomycosis, blastomycosis, cryptococcosis, and aspergillosis. The threat of bioterrorism with airborne infectious agents is also briefly presented.