Epidemics don’t necessarily decline because everyone has been infected, but also end because there aren’t enough infected people left to sustain transmission
From influenza to plague, the number of cases in a real epidemic often rises exponentially at first. After a while, the disease reaches a peak level, and then the number of new cases starts to decrease. When McKendrick and Kermack began their research (1927, in a paper titled ‘A Contribution to the Mathematical Theory of Epidemics), people generally gave two possible reasons for the decline. Either the epidemic faded away because the infection had become less potent over time, or because there were no susceptible people left – everyone had been infected and either died or become immune.
In their model, McKendrick and Kermack assumed that the pathogen stayed the same throughout the epidemic; the infection did not weaken over time. And yet the model still produced an eventual decline in cases. When the pair compared the model to the 1905 outbreak of plague in Bombay, the predicted number of cases matched the real disease level.
So was the decrease in infection caused by a lack of susceptible people? Apparently not: in the model, there were always some susceptible individuals remaining at the end of the outbreak. McKendrick and Kermack had demonstrated that epidemics don’t necessarily decline because everyone has been infected. They can also end because there aren’t enough infected people left to sustain transmission. Once enough people are immune, infected individuals are unlikely to meet another susceptible person, which means that they generally recover before infecting others.
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It's the idea that people living close to nature tend to be noble. It's seeing all those sunsets that does it. You can't watch a sunset and then go off and set fire to your neighbor's tepee. Living close to nature is wonderful for your mental health.
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But we can easily extend this hypothesis [that nature has beneficial effects on the physical, cognitive and emotional well-being of individuals] to the conservation of biodiversity. [Ecologists] refer to the extinction of the experience of nature, which they have mainly applied in urban areas. The idea is as follows: from generation to generation, young people live less and less in contact with nature (because there are fewer of them and because their lifestyles limit such contact), at the very moment they are building their identity. The part of their identity that integrates their intimate relationships with their natural environment would therefore diminish from generation to generation. Not because of a lack of education, but mainly because of a decline in opportunities and desires to experience nature without constraint, freely and in their own personal way.
The consequences of this decrease appear in adulthood: with a weaker environmental identity, they are less in demand for nature in their daily lives, they integrate it less in their actions. (...) But if we do not collectively take biodiversity into consideration in our lifestyles, then we will suffer.
An inefficient virus kills its host. A clever virus stays with it.