There are two causes of diseases - Immediate and contributory.
The immediate cause is the direct cause of the disease such as attack by a virius or bacteria.
Contributory causes includes all those causes which indirectly help in leading to the development of the disease. For example - Low immunity, lack of clean drinking water, crowded living conditions, poor nutrition, poverty etc.
Contributory cause by itself does not cause the disease, but will increase the chances of a person falling sick when exposed to the pathogen.