Depression is intimately more related to the HIV infection. Patients with MDD may tend to have increased risk-taking behaviour and the substance use. HIV transmission or acquisition is more likely to the result from risky behaviour in the depressed individuals or mentally distressed individuals than in those who are not depressed. Risky behaviour includes the increased number of lifetime sex partners, having sex with the injection drug users, having sex while intoxicated. Psychiatric patients with the depressive disorders who are substance abusers are said to be at the higher risk for HIV infection than are the psychiatric patients with another diagnosis. HIV infection increases with the risk of depression. While depressive symptoms may also appear as a reaction to a new diagnosis of the HIV infection, there is also evidence that advancing the HIV infection increases the depression directly. A proposed mechanism of the interrelationship between the HIV infection and the MDD might be a decrease in the cobalamin in the brain which is associated with the HIV infection, which further increases the risk of MDD. This is also associated with the suppression of the natural killer cells and CD8 T cells, thereby worsening the HIV infection. Another suggested mechanism is that the HIV infection increases the risk of depression and psychiatric issues by reducing the serotonergic transmission in the brain.