Depression is one of the most important causes of disability and loss of useful life of people around the world. Acute respiratory infection caused a large number of severe illnesses and deaths of the world and most of these due to viral infections, which is estimated more than 80% of respiratory infections. Detection of viruses by immune pathogen recognition receptors activates the intracellular signaling cascade and eventually cause produces interferons. Inflammatory process begins with secretion of interferons and the expression of interferon-stimulated genes. One of the most important of these genes is indoleamine-pyrrole 2,3-dioxygenase (IDO), which plays a major role in tryptophan catabolism. IDO is an intracellular monomeric enzyme that is also responsible for breaking down and consuming tryptophan in the Kynurenine pathway. Increased inflammation has been linked to decrease tryptophan concentrations and increase kynurenine levels. We tried to explain the role of inflammation by viral respiratory infections in causing depression.