Our immune system faces a monumental task in trying to prevent harm from foreign materials, viruses, fungi, tumours and chemicals, as well as the more common bacteria. After recognising an invader (called an antigen), the T-cell lymphocytes begin to multiply rapidly because they assume that there may be more 'enemies' around and so more lymphocytes may be needed to recognise them.
Next, the B-cell lymphocytes begin to produce chemicals, such as antibodies, which can attack and destroy or neutralise the antigens. As a sort of back-up, lymphocytes use phagocytes which simply 'eat' the attackers (the name literally means 'cell eaters'). The Natural Killer cells work independently much of the time and are able to destroy tumour cells directly. The more active your Natural Killer cells are, the less chance you have of a cancer developing, because your immune system is more likely to recognise and destroy a cell that has turned malignant before it can start dividing wildly and become a tumour. On the other hand, 'soldiers' of the immune system must recognise what parts rightly belong to our body and not attack them. In order to do this, we have a group of 'suppressor' cells which inhibit the immune response when it faces a 'natural' substance. If the suppressing mechanism fails, our immune system may attack and destroy our own tissues. Auto-immune diseases such as Multiple Sclerosis are caused by this phenomenon.
The human body is a very attractive home for innumerable bacterial, fungal and parasitical organisms. While some of them may leave us alone and co-exist in a symbiotic relationship with our bodies, the majority of them are able to make most of us very ill, indeed in some cases they can kill.
Probably one of the most frightening aspects of bacteria and viruses is the fact that in many cases they work synergistically with one another. One instance of this is the interaction between Candida albicans and the virus responsible for toxic shock syndrome, whose capacity to do harm is increased as much as a thousandfold by the presence of Candida at abnormally high levels. The more factors there are to load our immune system, the more likely we are to become victims of an assault from enemies which can overcome our depleted defences. This is why untreated allergies can predispose people to infections and vice versa.
If the medium in which they take up residence is favourable enough, bacteria and viruses can multiply at an astonishing rate. If it were not for the reaction known as the 'immune response', each organism that grows wild may well cause an epidemic. We have known about immunity for a long time - Greek physicians noted in 500 BC that those who recovered from certain infectious diseases never fell victim to the same problem again.