Lovesickness describes the informal syndrome

Syndrome

In medicine and psychology, a syndrome is the association of several clinically recognizable features, signs , symptoms , phenomena or characteristics that often occur together, so that the presence of one or more features alerts the physician to the possible presence of the others...

 of rejected or unrequited love

Unrequited love

Unrequited love is love that is not openly reciprocated or understood as such, even though reciprocation is usually deeply desired. The beloved may or may not be aware of the admirer's deep affections...

 and covers physical as well as mental symptoms. It is not to be confused with the condition of being lovestruck.
Almost all humans suffer from love sickness one or more times in their lives. Although typically harmless it can lead for some personalities to serious physical or mental sickness

Disease

A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...

, suicide

Suicide

Suicide is the act of intentionally causing one's own death. Suicide is often committed out of despair or attributed to some underlying mental disorder, such as depression, bipolar disorder, schizophrenia, alcoholism, or drug abuse...

 or death

Death

Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....

. In psychology

Psychology

Psychology is the study of the mind and behavior. Its immediate goal is to understand individuals and groups by both establishing general principles and researching specific cases. For many, the ultimate goal of psychology is to benefit society...

, love sickness is seldom acknowledged.

When a person is "love sick," they can't think of anything but the object of their desire. Being love sick generally happens to younger people who have little or no experience with an actual consensual loving relationship. Immaturity paired with hormones and strong emotions may be the cause of love sickness. Poetry, songs and Shakespearian plays are chock full of the heady expression of love sick people, pining away for the ones they love.

Symptoms

• Clammy hands and dry mouth are a symptom of being love sick.
• When the lover is not nearby, the love sick person may feel bereft, depressed, moody, tearful, anxious and preoccupied. The love sick person may also experience loss of appetite.
• A rapid heartbeat and other common effects of great excitement may occur upon seeing the lover, more so than usual.
• Confusion may result from anxiety
  Anxiety
  Anxiety is a psychological and physiological state characterized by somatic, emotional, cognitive, and behavioral components. The root meaning of the word anxiety is 'to vex or trouble'; in either presence or absence of psychological stress, anxiety can create feelings of fear, worry, uneasiness,...
  
  around the lover. Stuttering, staring and clumsiness are all aspects of this symptom.
• Sexual arousal, desperation and extreme desire are symptoms that can build into an obsession with the lover.

Duration

Despite its intensity, love sickness is a passing state of mind, lasting anywhere from a few days, to a few months. It's difficult to retain the intense level of emotion that love sickness awakens in the body, often the feelings lead the afflicted to either express his/her feelings or as time passes, the feelings lose their power as the reality of getting to know the real person takes away from the fantasy.