In contrast, with infectious disease, there is an imbalance between the threat posed by microbes and host defenses. Although the stress response is adaptive in a short time frame to allocate resources during a crisis, if the stress response is persistent, rather than cyclic, it further increases vulnerability to disease. The most common sequence of disease begins with a vulnerability and an exposure to one or more stressors. As a result of these and other vulnerabilities, the microbe more easily penetrates the host's defenses and an initial infection may then occur. Although infection may occur from microbes that are always present in the environment, a greater number of organisms or more virulent organisms further increase risk.
Acute infections are most noteworthy in general medicine.
Some injury in infectious disease is a result of toxic products or direct cell injury, but a significant amount of injury is a result of host defenses gone awry in response to the infection. Neural injury may occur by a variety of mechanisms, which include vasculitis, direct cell injury, toxins, inflammation, cytokines, autoimmune mechanisms, incorporation of parasite DNA into host DNA, and excitotoxicity. Injury to associative centers more commonly causes cognitive symptoms, while injury to modulating centers more commonly causes emotional and allocation of attention disorders. Psychiatric syndromes caused by infectious disease most commonly include depression, OCD, panic disorder, social phobias, variants of ADD, episodic impulsive hostility, bipolar disorders, eating disorders, dementia, various cognitive impairments, psychosis, and a few cases of dissociative episodes. In the collective database of patients demonstrating psychiatric symptoms in response to infectious disease, the majority of the cases has been infected by ticks.