Conclusion

From the above description of the variety of pathogens and the way they live and cause disease, it can be appreciated that the immune system is faced with an enormous variety of problems when trying to protect the body from disease caused by all the different types of pathogens. There is one additional factor that further challenges the immune system and increases the complexity of the immune responses required. Pathogens have coevolved with the immune system and have developed survival strategies to counter attempts to eliminate them. It is obviously in the pathogen’s best interests to survive in the host, and natural selection occurs so that pathogens with an improved ability to survive and multiply within hosts will have a selective advantage and become more common. The evolution of some pathogens seems to have been strongly influenced by the need to evade the immune response; for instance, cytomegalovirus (a cause of pneumonia) has devoted 30% of its genome to subverting the immune response against it. The immune system has accordingly had to evolve an equally complex variety of mechanisms to deal with the wide range of threats posed by different pathogens.