According to Longman Dictionary:
Illness is a disease of the body or mind, or the condition of being ill. Disease is an illness, which affects a person, animal, or plant. Microbe is an extremely small living thing, which you can only see if you use a microscope. Some microbes can cause diseases.

Illness is when microbes broke your immune system.
Why are people ill?  As I said illness and microbes have connection. Microbes include viruses, bacteria, protozoa and some fungi. If the harmful microbes come into your body through the air, food or cut they will make you ill. How they can do this? Very easily, they eat your body cells and make poisonous substances-toxins. Microbes reproduce really fast and more toxin mean more dead cells and then is your body ill.

But don’t be sad because you have immune system. Immune system should be your own army in your body, which has to protect you. The white blood cells play very important role in protecting your body. White blood cell called phagocytes eat microbes. Other makes antibodies, which stop microbes and the last ones make antitoxins, which neutralize the toxins. So phagocytes can eat them easily. When your mum tells you: “Eat the vegetables and fruits,” it’s a good advice, it isn’t joke. There are lots of important things (vitamins, carbohydrates etc.), which you and your immune system need in fruits and vegetables.
The best way how to keep your body healthy is balanced diet. Viruses are kind of microbes and the simplest life forms. They are very small (smallest then 0,0003mm) so they can go through ordinary filters. They live in cells. They cause measles, polio, flu, mumps, chicken pox, and hepatitis B.
Bacteria have plant origin. They have a lot of shapes. Different bacteria, different shape. They are big about 0,001mm. We can see them with light-microscope. Bacteria cause typhus, whooping cough, syphilis, pneumonia, diphtheria, and gonorrhea. Protozoa are mono cells organisms. They cause malaria, sleeping sickness, disease of liver, toxoplasm (I couldn’t find English so I create it, in Slovak it should be toxoplazmoza), trichomoniasis. Fungi are something between mushrooms and bacteria. They cause athlete’s foot, ringworm, thrush.

- The 5th most horrible illnesses through the agesThe top 5 most horrible illnesses through the ages:
· The 5th – Malaria -characteristics: damaging red blood cells, high temperature>death.
- It kills about 1 million people every year.
· The 4th – Tuberculosis (TBC)- characteristics: making junctions in lungs or disintegration of tissue with high temperatures, coughing out with blood>death.
· The 3rd – Syphilis –characteristics: hard ulcer on genitals, damaging of brain or hearth, blindness, paralysis.
· The 2nd – Tetanus –characteristics: muscles become hard, convulsions and cramps, grips
- Really horrible when you can’t move only wait for death
· The 1st – AIDS (acquired immunodeficiency syndrome)- characteristics: destruction of immune system, incubate time-about 9 years. AIDS killed about 28 millions people.
Other really harmful illnesses are cancer, SARS, Down’s syndrome, lepra, plague, mumps, dysentery etc. But If had to write about all of these it would be boring.