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Is there any evidence to say whether in the past viruses like the cold virus were less common? I wonder if high density living and globalisation mean that the cold virus mutates and spreads much more than in the past.

Perhaps in the past, owing to lower population density, slower and less frequent transport, viruses were effectively localised and isolated from the general population? Of course that might not be true, as perhaps those things have been offset by improvements in sanitation and nutrition? Perhaps virus frequency is always proportional to population size?

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  • $\begingroup$ Generally, media was less common, so the knowledge about a virus spreading anywhere else then where you live was less common. But yes, what you mention did lead to some viruses spreading faster and further. $\endgroup$ – skymningen Jun 30 '17 at 7:37
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    $\begingroup$ I remember reading a paper about the evolution of Bordatella pertussis (not a virus) and how it was influenced by increasing population density of humans several thousand years ago. I can try and find it again if you think it might provide an answer your question, or are you specifically asking about viruses and relatively recent history? $\endgroup$ – canadianer Jun 30 '17 at 7:41
  • $\begingroup$ I'd rather think about it in the way that humans are more common due to modernity and that the abundance of human pathogens is a consequence of this main effect. $\endgroup$ – AlexDeLarge Jun 30 '17 at 7:50
  • $\begingroup$ @canadianer The core question is about viruses like the common cold as they spread quicker than other diseases, but anything which can contextualise that over the long term is appreciated. $\endgroup$ – inappropriateCode Jun 30 '17 at 7:57
  • $\begingroup$ @AlexDeLarge Surely then that's demonstrable somehow? If viruses correlate with population size, or whether population density and connectivity affects this? $\endgroup$ – inappropriateCode Jun 30 '17 at 7:59
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Short answer: there are a lot of viruses, and they're all different. The answer to your question is going to be "it varies by virus". Some of the ways population density and urbanisation affect virus transmission are summarised very briefly below.

Population density

Any pathogen's ability to sustain itself relies on every infection causing, on average, at least one other infection. This is called the basic reproduction number or R0. The transmission rate of a pathogen depends on its transmission mode, but with some modes of transmission (like droplet transmission, a major route of transmission of viruses that cause the common cold) population density is important. There's an excellent explanation of density- versus frequency-dependent transmission here. This means that as people live at higher densities, the potential for transmission of some viruses increases purely because of that difference in density, while it has little effect on other viruses.

Critical community size

However, transmission rate is only part of the story. Assuming infected people eventually become immune, a pathogen can burn itself out before new hosts are born or immunity is lost. Stochastic effects become more important in small communities, and there is a critical community size below which pathogens tend to go extinct. Small or isolated communities cannot sustain the virus causing measles, for example.

Urban environments and transmission (and vector-borne diseases)

Urban environments aren't just higher density, they are different in potentially important ways. For example, some viruses need to be transmitted between vertebrate hosts by arthropod vectors like mosquitoes. If you don't have the mosquito, you can't get the virus. Viruses like yellow fever virus, dengue virus and Zika virus are primarily transmitted by Aedes aegypti, which nowadays breeds preferentially in urban environments, so as urbanisation increases the potential for transmission of these diseases also increases. Without adequate sanitation, the potential for faeco-oral transmission also increases at high population densities, for example. Things like sewage treatment, air conditioning and exposure to wildlife all affect transmission in important ways.

So, basically, it's complicated.

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