Dear Student,
The external causes such as microorganisms are considered as immediate causes and the factors which do not cause the disease themselves but provide an environment or favourable conditions for the development of disease are considered as contributory causes. As the name suggests, they contribute to developing a disease.
The primary cause of diseases is microorganisms such as a virus, bacteria, protozoa, etc. Poor nourishment, unhygienic practices, poverty, lack of proper food and unhygienic environment are the main reasons for the entry of a microorganism in the human body. Microorganisms can enter the human body through contaminated food and water. Moreover, genetic constitution of an individual also makes him susceptible and resistant to diseases. An individual with an effective immune system will be more resistant to diseases and an individual with a weak immune system is more susceptible to diseases. Thus, a disease cannot be caused by a single factor but it is an outcome of the interaction between several factors. Therefore, all diseases will have immediate causes and contributory causes. Also, most diseases will have many causes rather than one single cause.
Regards