Return of the old infectious diseases

Despite the remarkable natural defence mechaifisms that have evolved, they have not always been successful and infections have, on occasions, produced catastrophic effects on the human population (Box 17.6). Two examples are the plague pandemic of the mid-1300s (Box 17.7), and the influenza pandemic of the early 1900s. This is due, in part, to the ability of the pathogen to avoid detection or killing by the immune system. 

Some of the reasons for the return are as follows (i) new breeding grounds for the insects that are vectors for some pathogens (ii) antigenic drift in viruses and bacteria (iii) resistance to antibiotics (iv) a decrease in the effectiveness of the immune system due to the presence of other more chronic infections, poor nutrition or stress (v) expansion of air travel. 

One example is the mosquito. Conversion of forests into farmland in many developing countries produces places that accumulate water which are ideal breeding grounds for mosquitoes. Global warming may increase the size of areas that are warm enough for mosquitoes to breed (Box 17.8). 

Box 17.6 Strategies employed by pathogens to evade detection or killing by the immune system 

Microorganisms have evolved a wide range of compounds that interfere with defence mechanisms provided by the immune system. 

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