For example, hives can be brought on by a physical allergy or a psychologic reaction. Depression can suppress the immune system, making a depressed person more susceptible to certain infections, such as those by the viruses that cause the common cold. Stress, therefore, can cause physical symptoms even though no physical disease may be present. The body responds physiologically to emotional stress. For example, stress can cause anxiety, which then triggers the autonomic nervous system and hormones such as adrenaline to speed up the heart rate and to increase the blood pressure and amount of sweating.
Stress can also cause muscle tension, leading to pain in the neck, back, head, or elsewhere.
The emotional disturbance that triggered the symptoms may be overlooked when the patient and the doctor assume that they were caused by a physical disease. Many fruitless diagnostic tests may be done to uncover the cause of the fast heart rate, headaches, backaches, and so on. Psychologic factors can also indirectly influence a disease's course. For example, some seriously ill people deny having a disease or deny its seriousness. Denial is a defense mechanism that helps reduce anxiety and makes a threatening situation more tolerable.