Unlike cancer or diabetes , an infectious disease is a disease that can be passed on to others. This is because the 'infectious agents' that cause the disease by attacking the body's cells can move from one environment to another - from dirty water into one's body, for example. Agents that cause infectious diseases are known as ' pathogens '. The three most important kinds of pathogens are viruses , bacteria and protozoa .
Viruses are the smallest of the three.
They are not themselves made of cells. In fact, they need to enter the cells of people and animals to survive and reproduce. Unfortunately, in doing so, many viruses harm our bodies. So while they make their way into our cells only to keep on living, the effect is like an attack on our bodies. Bacteria and protozoa are different.