Illness is caused by the spread of viruses, bacteria, and germs. This happens directly and indirectly. Illnesses are preventable but can be highly contagious. Read on to know the main causes for the spread of illnesses. Direct Causes The easiest causes for the spread of illness are the direct causes. The direct causes include person-to-person contact, animal-to-person contact, mother to unborn baby contact. Person-to-Person Contact Illness can spread easily from an infected person to an ...