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Blood clot

A mass of fibrin, platelets and red blood cells which form a solid mass and stop blood loss at a wound.


A structure with a particular function which is made up of different tissues.

How pathogens cause disease


Once you are infected bacteria grow and reproduce inside the cells of your body. As they grow and divide, many bacteria make toxins (poisons). These toxins can damage your cells. Some bacteria damage your cells directly as they grow. Disease symptoms such as a high temperature, headaches and rashes can be caused by the damage and toxins or by the way your body responds to the damage and toxins produced by the bacteria.


Viruses cannot reproduce outside of the cells of their host. Once you are infected, viruses take over the genetic material of your cells. In this way they direct your cells to make copies of the virus until eventually the cell bursts, releasing many more viruses to infect other cells. In this way viruses kill infected cells, spread and damage body tissues. The way your body responds to the bursting of the cells causes the symptoms of the disease.

Animation showing how a virus infects a cell.

Preventing infection

As you have seen, there are pathogens all round us and many different ways they can be spread from one person to another. Your body needs to be able to stop these pathogens from gaining entry. This part of the immune system is called non-specific immunity because it is present all the time and not activated in response to a particular pathogen.

So you think you are clean?

Part of body Bacteria
Head (scalp) 1,000,000 /cm2
Surface of skin 1000 /cm2
Saliva 100,000,000 /g
Nose mucus ('snot') 10,000,000 /g
Faeces over 100,000,000 /g

The table shows the numbers of bacteria that are living on you and inside you all the time. It is easy to see why your body needs barriers to prevent pathogens entering and causing a serious infection.


Your skin is possibly the most important barrier to prevent infection entering the body. Cuts and grazes can break this barrier and there are systems to automatically repair any damage. Your skin is a specialised organ that not only protects but also senses the environment and helps to regulate your body temperature.

Breaking the barrier

The outer layer of skin forms a tough barrier to infection that is effective as long as it is intact. Cuts, grazes, burns and hypodermic syringes are all ways that this barrier can be broken and pathogens gain entry into the body. To prevent this happening, the skin can repair itself when any damage occurs.

Click on the numbers below to see the steps of repair when the skin is cut.

  • Stage 1
    a sharp object cuts the skin.
  • Stage 2
    blood enters the wound. A blood clot quickly forms to seal the cut and prevent blood loss and entry of pathogens. The clot dries to form a scab.
  • Stage 3
    new skin grows to repair the damage caused by the cut. After a week or two the scab falls off.