According to the Free Dictionary, the prodromal stage of an infectious illness is the period between invasion of the body by an organism and the point at which the characteristic symptoms of the illness appear. A person in the prodromal stage of an infectious illness often displays non-specific symptoms, such as fatigue or malaise, Wikipedia explains.
Many infectious illnesses include prodromal symptoms or a prodromal stage, according to Wikipedia. For example, infections caused by the herpes simplex virus, or HSV, usually include prodromal symptoms of itching, tingling and pain before the blisters that are characteristic of herpes infection occur. Measles infections usually have a prodromal period that includes symptoms such as a fever, runny nose and red, itchy eyes. Over 35 percent of children infected with the varicella virus that causes chicken pox have a prodromal fever before the characteristic rash appears.
Non-infectious diseases often have prodromal stages, we well. According the University of Colorado's Adolescent Development and Treatment Program, psychiatric illnesses and psychoses also display a prodromal stage. For instance, many people exhibit symptoms such as unusual thinking, extreme sadness, withdrawal, anxiety, excitability, sleeplessness and a decrease in overall functioning for several days to several years before experiencing a psychotic episode.
Migraine sufferers, too, often experience prodromal symptoms for several hours before an attack, according to WebMD. These include feeling unusually energetic or sleepy, excessive thirst, cravings for certain foods or a need to urinate more often than usual. These symptoms generally are followed by an "aura" that involves changes in vision or speech, confusion or trouble concentrating before the actual headache begins.