Infectious diseases are caused by the transmission of pathogens, which are microorganisms such as bacteria and viruses. Pathogens can be transmitted by direct contact - horizontal and vertical transmission - and by indirect contact - vehicle-borne transmission and vector-borne transmission.
Pathogens are microorganisms that cause infectious disease. Pathogens are mostly bacteria but some are viruses, fungi and protoctists.
Bacteria come in many shapes and sizes, but even the largest are only 10 micrometres long (10 millionths of a metre).
Bacteria are living cells and, in favourable conditions, can multiply rapidly. Once inside the body they release poisons or toxins that make us feel ill.
Viruses are many times smaller than bacteria.
Viruses can only reproduce inside host cells, and they damage the cell when they do this. A virus can get inside a cell and, once there, take over and make hundreds of thousands of copies of itself.
Direct contact means that the disease-causing microbe is passed from one person to another when their bodies touch in some way.
Vertical transmission happens when microorganisms pass from a mother to her unborn baby through the placenta. German measles and HIV can be passed on this way.
Horizontal transmission happens when microorganisms pass from one person to another by touching, kissing or sexual intercourse.
Examples of Horizontal Transmission
Type Of Contact ~ Bacterial Disease ~ Viral Disease
touching bacterial gastroenteritis chickenpox
Indirect contact happens when microorganisms are carried to a person in some way, instead of by actual body to body contact.
Vehicle-borne transmission involves an object carrying the disease-causing microorganism.
Vector-borne transmission involves an animal such as an insect. For example, malaria is transmitted by mosquitoes, dysentery by houseflies and plague by fleas.
Most pathogens have to get inside our body to spread infection. Once they are inside, the body provides ideal living conditions, including plenty of food, water and warmth. Standing in their way is our body's immune system - the body's co-ordinated response to the invading pathogens.
How Does The Body Fight Back?
Passive defences are those that are set up to stop bacteria or viruses entering the body. They act as roadblocks. They are found in those places where the invaders are most likely to try to the body.
Invaders try to get inside us via five main areas:
- The Skin
- The Respiratory System
- The Eye
- The Digestive System
- The reproductive System
The skin covers the whole body, protacting against physical damage, microbe infection and dehydration. Its dry, dead outer cells are difficult for microbes to penetrate, and the sebaceous glands produce oils that help kill microbes.
The skin is a very good, tough barrier to infection so long as it does not get damaged.
When it is cut or punctured the flow of blood will carry out some invaders. Then when it clots it stops others getting in.
The Respiratory System
The respiratory system (or gaseous exchange tract) is protected in several ways. Nasal hairs keep out dust and larger microorganisms. Sticky mucus traps dust and microbes, which are then carried away by cilia (tiny hairs on the cells that line the gaseous exchange tract).
The airways all the way down into the alveoli in our lungs give a perfect way to invade.
Fortunately they are lined with cells that can produce mucus in which dust and microbes get trapped. This mucus is then brought up towards the mouth by the wave-like action of the cilia attached to the cells.
Contains very delicate tissues that can be easily crossed. To give protection our tears will wash out some invaders and releases an enzyme that kills bacteria.
The Digestive System
Many microbes enter through our mouths on our food.
However they face a difficult time! Our saliva contains an enzyme which attacks bacterial cells. Then our stomach contains hydrochloric acid and more enzymes.
Most microbes will be killed. So not a pleasant trip!
The Reproductive System
Apart from the thin skin present around the penis and vagina, there aren't many defences to microbes. So quite a few microbes are able to cause diseases if they try this route of attack.
Once the body's passive defences are breached there is another line of defence. The active system of cells go and deal with the invaders directly. This is the immune system.
The most important of the cells in this defence system are the white blood cells
White blood cells are obviously carried around in the blood, but they can also crawl out of blood vessels and get to any cell in the body. They can go anywhere!
The immune system cells act in 3 ways:
Consume The Invaders
The white cells extend their cell membrane around the invader. Eventually it is taken inside the cell by endocytosis and enzymes are secreted to destroy it. The white cell then absorbs what is left.
Other white cells produce antibodies when they meet an invading microbe. These special protein molecules stick to the microbes and mark them as 'foreign' invaders.
The microbes are either killed or other white cells come along and consume them.
The beauty of antibodies are that once they have been made your body is always ready attack that same invader on another occasion. You have developed a natural immunity to it.
Yet other white cells produce antitoxins. These are like an antidote to the toxin poisons made by some bacteria. The antitoxins stick to the toxins and make them harmless.