Plague Nation - Infectious Diseases

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INFECTIOUS DISEASES

More people die each year from infectious diseases than from any other cause, but the deadly agents that cause this devastation are microscopic. They are called PATHOGENS and they include BACTERIA, VIRUSES and PROTOZOA

bacteria

virus

protozoa

You've probably been ill recently yourself and you might have had to stay at home because you had a temperature or a stomach upset or a sore throat. If you did it's probably because you were infected by one of these microscopic organisms. If it was a cold it would have been caused by the ADENOVIRUS, if it was the flu then it would have been caused by one of the strains of the influenza virus. Other infectious diseases include tuberculosis, HIV, polio, diptheria and malaria.

Diseases like this are called infectious because they can be passed on to other people. So, that cold that you had, might have been passed on to you by that coughing and sneezing person sitting next to you on the bus. And you may then have passed it on to someone else ... and so on. Fortunately the human body is generally very adept at coping with these microscopic invaders - it's got several lines of defence.

THE SKIN AND INNER LININGS
The skin is pretty good at keeping out invading pathogens as are the linings inside the body such as those covering the respiratory tract. But these are also quite fragile, it's easy to get a cut on the skin and that's when the pathogens can sneak in and seek out the cells in the body that it needs to reproduce.

WHITE BLOOD CELLS
It's the job of white blood cells to attack these invaders when they manage to get past the first line of defense. They gather around infected areas and work to stop the infection spreading. However sometines the invading pathogens are too strong for the white blood cells.

T CELLS and B CELLS
T cells are a particular kind of white blood cell which can recognise and destroy diseased cells. B cells don't do any destroying but they can recognise infected cells and leave a mark which the other white blood cells recognise as a cell needing to be destroyed.

This ability of the body to fight off infection is known as IMMUNITY. It is also boosted by the presence of antibodies which are produced after the body has been exposed to a specific virus or bacteria. They stay in the body for a period of time and help prevent the body developing a disease even if exposed to the pathogen.

Sometimes however, the bacteria, viruses or protozoa are too strong for the body to cope with on it's own and this is where medicine has stepped in. A number of methods of coping with infectious diseases have been developed, one of the most important being VACCINATION which helps to build up the body's natural defences against disease. This has enabled a number of diseases to be brought under control but these bacteria and viruses are pretty clever and as much as we want to kill them off they want to survive. New diseases are emerging all the time which threaten the health of the world. The HIV virus, for example, has only relatively recently been identified but millions of people are infected with it across the globe and millions are dying each year and we still do not have a vaccine or cure for it.

Other older diseases which people thought were generally under control are beginning to re-appear. One of these is TUBERCULOSIS which kills 2 million people each year.

Part of the reason for these diseases being able to survive is the fact that they can change to make themsleves more powerful. For example, scientists researching the 2002-03 Sars epidemic found that the early version of the virus only infected about 3% of those who came into contact with it. This strain of the virus was almost the same as the one found in animals. But as the virus began to be spread from human to human it mutated, it adapted itself for its new found host, human beings. As it spread it learnt how best to move between people. So within a few months it infected 70% of the people who came into contact with it. Soon the virus was able to spread itself across the world.

So the fight against infectious diseases is a constant and global one and for humankind to win the battle EPIDEMIOLOGISTS have to keep researching how they are spread and how they are caused and scientists have to continue to search for vaccinations and cures.