What is happening in the body when a person has tremors?

Fast muscle activity: Tremor is are rhythmic muscle contraction often involving opposing muscle groups. It is often involuntary and can be often suppressed. Tremors affect hands, arms, eyes, face, head, vocal folds, trunk, and legs. At times the tremor is seen in an healthy person (fear, cold temperature, fever...). Many tremors are treatable and should be evaluated by a neurologist.

Multiple causes: Assuming you have whole body tremors, the possibilities include fever, fear, hyperthyroidism, alcohol or drug withdrawal and some medications may cause it.