For disease (illness, sickness) there is no simple and unambiguous definition. Also because there are so many different diseases.
It is a collective term for disturbances in the normal functioning (body and mind).
A change with harmful effects (to tissue or function).
Dysfunction, imbalance or discomfort that upsets feeling good (or healthy).
A process with a beginning, progress and end. (Sometimes the end.)

Ziekte&symptomenBefore symptoms appear we are already sick. The symptoms merely indicate that your body / mind cannot take it anymore now. The symptoms can help an expert to determine what is wrong. A doctor makes a choice between the known patterns of the existing signs and symptoms. Signs are by the physician observable and measurable data, often obtained through additional research. Characteristic symptoms can refer to a known syndrome or disease.
A syndrome is a disease image: a series of always frequent clinical together occuring (for a counselor or physician visible and recognizable) signs or symptoms. A symptom is a characteristic or complaint, a characteristic at which the problem is identifiable.
The task of the physician is the identification of the syndrome, the diagnosis, and, if appropriate, the recommendation of a treatment or cure.

To detect a disease can be examined the (deviation in terms of) (body) temperature, weight, blood pressure, respiration, (skin and facial) color, feces, urine, pain, pulse, dehydration, vomiting, lack of appetite, drowsy or lethargic, problems with movement (of limbs, back..), mucus, snot, crying.

The symptoms can be acute: sudden occurrence, an attack ;
or chronic: persistent or prolonged illness, inflammation etc.
Sudden worsening, intensification and / or expansion of a disease or one or more of the symptoms (e.g. rash, fever, etc.) is called exacerbation.
And recidivism refers to a recurring disease or symptom.

Therapy, assistance, treatment or cure can be found at massage, acupuncture, bloodletting, surgery, medicine (herb, organic, mineral), diet, therapies with gong, candle,..

Preventively, to maintain good health and fitness are important: good hygiene, vaccination, lifestyle, avoiding dangerous and foreign substances.

Disease and symptoms: you do not have to be sick to get better. (Hans Mes)
Nobody is sicker than the man who is ill on his day off. (Bill Vaughan)