Stress can be thought of as a negative emotional state occurring in response to stressors that are perceived to be taxing or exceeding a person’s resources and/or their ability to cope with the situation. Stressors are events or situations that are perceived as harmful, threatening, and/or challenging. Stressors can influence one's health, an existing illness, one's medical treatment, and/or health-related behaviors. It's not only negative events that are harmful but positive events are quite stressful as well. For example, a wedding or graduation can be quite stressful for many people. All stressors can affect health and illness in a complex interaction of biological, psychological, & social factors.

Not only do major stressors such as divorce, separation, death, and marriage/relationship problems contribute to one's total amount of stress, but also daily hassles or everyday annoyances produce an accumulative effect to one's stress. This stress is linked to both psychological distress & physical symptoms, such as back pain & headaches. The quantity of daily hassles has been found to be a better predictor of illness than the quantity of major life events. In addition, these minor stressors are cumulative and contribute to the stress of major life events.

Other factors that contribute to stress include conflict, crowding, crime, unemployment, poverty, unfinished tasks, discrimination, inadequate health care, and substandard housing. In addition, culture shock or acculturative stress can often negatively affect people who leave their country of origin to live another country.

The direct effects of stress can lead to emotional or physical symptoms, illness, or disease and can include a wide range of health problems from headaches to heart attacks. On the other hand, the indirect effects of stress can prompt the use of maladaptive behaviors such as alcohol, coffee, and cigarettes, which in turn can interfere with cognitive abilities such as attention, concentration, & memory.

Stressors activate the fight or flight response and if prolonged can impair the immune system and lead to harmful physical effects such as high blood pressure and heart disease. It has been found that people with Type A behavior react more intensely to stressors. Type A behavior includes 1) an exaggerated sense of time urgency, 2) excessive hostility, anger, and irritation, and 3) intense ambition and competitiveness. Of these, it has been found that hostility is more likely to lead to heart disease. Hostility is defined as the tendency to feel anger, annoyance, resentment, and contempt, and to hold negative beliefs about human nature in general.