You wake up one morning with a throbbing head and a sore throat. Then you start to cough. How did you get sick?

In this activity, you will look more closely at the causes of disease in humans.

Why do people get sick? Scientists through the years have developed theories regarding this. After the invention of the microscope, scientists were able to find and identify micro-organisms called microbes. The microbes that cause diseases are called pathogens or germs.

This image shows germs under a microscope.

 Germs live in toilets, in air conditioners, and on washcloths. If these are not cleaned regularly, the number of germs may increase. Some people may not experience health reactions from certain germs, while others do. Pathogens are disease-causing microbes. They include some bacteria, viruses, fungi, and parasites. You will look more closely at two of these pathogens below.
     
Germs are not the only things that cause disease. Some diseases are caused by poor environments; for example, air pollution may cause damage to the lungs and lead to lung disease.

Diseases may also arise from a combination of factors that include diet, lifestyle, genetics, germs, and stress. For example, have you ever noticed how often you get sick when you are worried about school work or your job? Often, when people are stressed, they do not eat properly and may stop exercising. These diet and lifestyle choices combine with stress to make people more likely to get sick.  

Bacteria

Bacteria are in your mouth, on your skin, in the food you eat, and in the air you breathe. You can’t see them because they are very small single-celled organisms that you need a microscope to see.

Some bacteria are beneficial to humans. Yogurt is made from milk and a bacterium that causes the milk to go sour. There's some evidence that some strains of yogurt can help boost the immune system and promote a healthy digestive tract.

Other bacteria are harmful to humans. They spoil food and cause disease. Both helpful and harmful bacteria reproduce quickly and form colonies when conditions are good.

Bacteria thrive in the following conditions:

1. Temperature — Most bacteria are active at warm temperatures. Refrigeration or freezing slows bacterial growth. Bacteria die when temperatures get too high.

2. Moisture — Bacteria need water. Many cannot reproduce without it.

3. Food — Bacteria thrive on sugar, protein, or fats.   

The above image shows examples of three different shapes of bacteria: cocci, bacilli, and spirilla.

A bacterial infection may occur when a harmful strain of bacteria gets on or inside the body.

Pneumonia, meningitis, and food poisoning are just a few illnesses that may be caused by harmful bacteria.    

A viral infection is a disease that can be caused by different kinds of viruses. A viral infection can occur in different places in the body; some viruses are mainly in the intestine, while others prefer the lungs and airways.

Viruses

Like bacteria, viruses can be found anywhere there are cells to infect — and humans are made of trillions of such cells. Viruses, such as the common cold virus, are much smaller than bacteria. They reproduce by taking over cells in the body. In this way, the number of viruses quickly multiplies and invades other body cells.