Viruses are sometimes called the hidden enemy - and you can understand why. They are microscopic particles, much smaller than bacteria, which can cause a number of common illnesses such as flu, chickenpox and the common cold.

Unlike bacteria, you don't really get 'good viruses' - they all behave pretty nastily, taking over host cells inside a plant, animal or human in order to grow and divide.

Once viruses have carried out this 'hijacking' they can reproduce to create more viral particles which can then spread to other cells.

Not all bacteria are the enemy. In fact, some of them are our allies. Most of them, like Lactobacillus and Bifidobacterium, are helpful, and live in our bodies to maintain normal function, taking care of things such as maintaining digestive health and breaking down food. In fact, people have more bacteria cells in their bodies than human cells.

Some bacteria, though, are not good for us, and can cause illness in humans. And because, whether friend or foe, you can't see them on surfaces (they are only a thousandth of a millimetre in size!), it's important to make sure your home is regularly disinfected.