Infectious diseases are illnesses that are contagious, meaning they can be spread from one person to another. Infectious diseases are caused by microorganisms, called pathogens, or what we commonly refer to as germs. Viral and bacterial infections are the most common causes of infectious disease. However, fungi and one-celled organisms called protozoa can also be responsible. Flu or influenza Non infectious diseases are illness that are non contagious and cannot spread through contact. Example is Allergy and blood pressure.