Some diseases attack very suddenly. We called these "acute diseases" ("acute" means "sharp"). These can be serious or not, but they usually make the patient feel quite ill. The acute disease ends with a patient either dying of the disease or recovering from it. Sometimes, a disease can continue for a long time. This is called a chronic disease (from 'chronos', meaning 'time'). If an organ of the body is so damaged that it can no longer work well, we now call this ailment an "organic disease".

Many organic diseases are caused by parasites which are small organisms that live within us and take nourishment from our systems. Parasites do damage to their hosts and are responsible for "parasitic diseases". Some parasites, like worms, are big enough to be seen with the naked eye. In some parts of the world, people can easily pick up hookworms when they walk barefoot. These worms travel into the intestines and live on blood. This disease can easily weaken a person and some of them die.

On the other hand, some parasites as so small that they are cannot be seen without a microscope. Such parasites are called microorganisms or germs. They cost the "germ diseases". Germs are carried in many ways from person-to-person, most especially when somebody coughs or sneezes. Sometimes they can get into our systems through food and drinking water. It is so easy for a sick person to communicate their disease to a healthy one. For this reason, such diseases are called "communicable diseases" or "infectious diseases".

If the germs of a certain disease are easily spread from person to person, it is called a "contagious disease". Germs come in many sizes and kinds. Relatively small germ cells are called protozoa and the most common "protozoan disease" is malaria. The malarial parasite lives in red blood cells and destroys them. Many people in the tropical parts the world suffer from malaria every year, which probably kills more people than any other disease.

Other germs are very simple plant cells called fungi or molds. Athlete's foot is an example of a "fungus disease".

Bacteria are even simpler than just germs. They are living things that can be seen only through an ordinary microscope. "Bacterial diseases" include tuberculosis, scarlet fever, tonsillitis and many other simple infections.

Finally, there are other germs too small to be seen under an ordinary microscope that they need to be checked under an electron microscope. These are the viruses that are responsible for the most familiar sicknesses like measles, chickenpox, mumps and the flu. These are also called "viral diseases".

There are other types of diseases that are not caused by germs. In fact, disease can be brought on by any physical or chemical injury. Too much sun can bring sunburn or sunstroke. Some chemicals that enter the body can be poisonous. Sometimes a harmless substance becomes poisonous to a particular person. It makes a person sensitive to a substance in food, pollen, or even some medicine. He is said to suffer from an allergy, which is a form of disease.

People in some lines of work sometimes run into particular risks of poison or other chemical damage. Some people who work in the paint industry (because of their exposure to lead) may absorb dangerous chemicals into their bloodstream. Lead poisoning is an example of an "occupational disease".

A person can also be sick because of what he has not taken into the body. Remember that the body requires a certain amount of good food and clean water to live. If the food or water lacks certain substances that the body requires, the body develops a "deficiency disease" like anemia (which is the lack of iron in the blood). In the absence of the necessary vitamins, diseases like crickets and scurvy develop. These are also known as "vitamin deficiency diseases".