An infectious diseases is scientifically proven disease caused by microbes that are pathogens (called as pathogenic microbial agents). These are of multiple types.

• Pathogenic bacteria, causes diseases such as plague, anthrax and tuberculosis
• Protozoa,causing diseases like malaria, toxoplasmosis and sleeping sickness
• Fungi, causes infections like ringworm, candidiasis or toxoplasmosis
• Pathogenic viruses cause diseases like influenza, yellow fever or AIDS, which are basically not living organisms and are not microorganisms

These agents can cause diseases in animals or plants or both. These are usually qualified as communicable diseases ( also called contageous diseases) as they have the capabilities of transmission from one person to many others.

This transmission can happen in multiple ways. Air, physical contact, food, blood transfusion, body fluids or through contaminated objects