An uneasy relationship
Populations and disease are intertwined
The spread of disease affects a population in the short term through ill health and death, but also in the long term by influencing the overall genetic structure of the population. One of the leading causes of death globally is HIV/AIDS. There were 1.6 million AIDS-related deaths in 2012, with many occurring in populations in low-income countries where treatment is less affordable.
The ravages of HIV/AIDS may affect the genetic make-up of populations. One genetic mutation, CCR5-delta 32, protects against HIV, helping people survive. It is therefore becoming more common – and more likely to spread in affected regions. Some scientists think that CCR5-delta 32 may have offered protection from other diseases in the past and that this is why it’s particularly common in Europe, where bubonic plague wiped out around a third of the population in the 14th century.
The behaviour of some diseases depends on population size. Measles statistics show that before a vaccine became available, larger cities were associated with an increased frequency of epidemics. This is probably because in smaller cities there were not enough susceptible people to allow the measles virus to circulate – people had already been infected and become immune.
Plants get sick too, of course. A fungal disease called ash dieback, which has been spreading through Europe since the 1990s, has killed between 60 per cent and 90 per cent of Denmark’s ash trees, leaving only a small population of resistant trees. Ash trees are important in woodlands because their canopies allow light to filter through to support life on the ground.