Bacteria, fungi and viruses are the major microbes which can enter both nasal and lung region. The present article gives an outline of different bacteria, viruses and fungi which causes respiratory tract infection (RTI). Streptococcus pyogenes, Bacterial rhinosinusitis, Diphtheria, Bacterial pneumonia, Pneumococcal pneumonia, Haemophilic pneumonia and Mycoplasma pneumonia, Tuberculosis, Pertussis (Whooping Cough) Legionnaires Disease and Q disease are the RTI caused by the bacteria. Infections like Histoplasmosis, Blastomycosis, Coccidioidomycosis, Aspergillosis, Candidiasis, and Mucormycosis are some of the examples of respiratory tract caused by fungi. RTI caused by viruses included influenza, common cold, measles, mumps, rubella, chickenpox, and syndromes like SARS and MERS. The present article discusses about the various organisms that causes respiratory tract infection. Keywords: Upper respiratory tract infection, lower respiratory tract infection, bacteria, Streptococcus pneumonia, Staphylococcus, Influenza, opportunistic infection, Aspergillosis, Histoplasmosis, Mucoromycosis