A biological factor is anything that affects the function and behavior of a living organism, according to Psychology Dictionary. These factors can be physical, physiological, chemical, neurological, or genetic. Biological factors are the primary determinant of human behavior and are routinely examined when studying the causes of mental illness.
WebMD states that the main biological factors implicated in mental illness are genetics, infection, brain defects or injury, prenatal damage and substance abuse. Due to the environmental factors which can influence the onset of mental illness such as trauma, abuse or stress, a person genetically predisposed to a mental illness may or may not finally express the illness.
Certain infections have been linked to brain damage and aggravation of mental illness symptoms. An example of such an infection is pediatric autoimmune neuropsychiatric disorder (PANDA); this affects children and is linked to obsessive compulsive disorder and some other mental illnesses, according to WebMD. The infection that triggers PANDA is caused by the streptococcus bacteria. Mental illness can occur due to defects or trauma to the brain as an adult or during fetal development. Long term substance abuse may cause anxiety, depression, or paranoia. Examples of other biological factors that can trigger mental illness are poor nutrition and exposure to toxins.