Every day the human body comes under attack from a range of microscopic invaders that cause disease. All kinds of defensive measures are in place to stop them. Skin and membranes form physical barriers. Fluids such as saliva, tears, and mucus provide chemical warfare. If these lines are passed, the immune system fights back. Armies of special cells target and destroy enemy attackers to make the body healthy again.
Body invaders
Pathogens are bacteria and viruses that cause disease. Most bacteria are simple and harmless, and some are helpful, such as those that live in the gut to help with digestion. However, some bacteria invade and damage body tissues. Viruses are chemical packages much smaller than bacteria that take control or body cells and multiply, causing illness and disease.
Bacteria
Bacteria are simple single-celled organisms that can multiply rapidly. A few can cause serious diseases by invading the body, and some release poisons called toxins.
- Bacilli: These rod-shaped bacteria often live harmlessly in the gut. Other bacilli cause illness, such as bladder infections and typhoid.
- Spirlla: Small spiral-shaped bacteria, called spirlla, come from uncooked shellfish or stale water. These cause stomach upsets and diarrhoea.
Viruses
Viruses reproduce by invading a body cell. The hijacked cell is turned into a factory where more viruses are produced. These are then released to infect more and more cells.
- Adenovirus: This virus can infect lungs to produce coughs, eyes to give conjunctivitis, and the digestive system to trigger diarrhea.
- Influenza: There are three main types of influenza virus. Types A and B can cause flu, especially in the winter. Type C usually causes a milder respiratory illness.
Fungi, protists, and parasites
Although most fungi grow in soil or rotting food, some live on or inside humans. Protists are simple organisms, some of which cause human disease. Parasites are other living things that live on or in our bodies.
- Athlete’s foot: This fungus, called Trichophyton, grows as a network of threads in damp skin, especially between the toes, it causes an itchy infection.
- Plasmodia: Single-celled plasmodia live inside mosquitoes. A mosquito bite can bring plasmodia into the human bloodstream where they infect red blood cells.
- Tapeworm: This parasite may live inside the intestines but not cause any symptoms. Tapeworms enter the body via undercooked or raw meat that contains tapeworm larvae.
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