An infectious microbe is able to cause a disease only if the immune system of the infected person is unable to put Proper defense against it. Many-persons have strong immune system or have acquired immunity against the pathogen or the intensity of pathogenic attack is quite mild. As a result, despite exposure to infective microbe, the person will not catch the disease.

Prevention is better than cure as a disease always causes some damage to the body, loss of working days, besides expenditure on medication. The important precautions for preventing occurrence of diseases include
(i) hygienic environment;
(ii) personal hygiene;
(iii) proper nutrition (balanced diet);
(iv) clean food;
(v) clean water;
(vi) regular exercise and
(vii) adequate relaxation. Everybody should also be aware of diseases and their mode of spreading (epidemic). A regular medical checkup is also earnestly required to stay healthy.

Syndrome is a group of symptoms, signs, physical and physiological disturbances that are due to a common cause. AIDS is also a complex of diseases and symptoms which develop due to failure of the body to fight off even minor infections. HIV that causes AIDS damages immune system of the patient by destroying T₄ helper cells (T lymphocytes). As a result, even small cold leads to development of pneumonia, a slight gut infection leads to severe diarrhoea and blood loss, likewise skin rashes develop into ulcers.

Disease symptoms are evidences of presence of diseases. Symptoms are in the form of structural and functional changes in the body or body parts. They indicate that there is something wrong in the body, e.g., wound with pus, oedema, cough, cold, loose motions, pain in abdomem, headache, fever. However-symptoms do not give any exact cause of the disease. For instance, headache may be due to some dozen different diseases. There may be problem of eye sight, blood pressure, some sort of stress (such as of examination, meningitis, pollution, etc.).