The entire logic of cities is commerce, interaction, transport, and exchange, whether commercially, socially, educationally, culturally, at work, at play, or in services.

The challenge is that noxious phenomena as well as beneficial ones thrive in such conditions. It's interesting to chart the topics topping lists of "urban problems" over the years, decades, and centuries, they're actually quite fluid and reflect overall trends, growth, decay, and stasis, fairly well. Transitions over the 20th & 21st centuries, from growth to decline to revitalisation to gentrification, and now, pandemic, areinstructive.

More critically, one comes to realise that cities don't exist as some ultimate goal or end of all human progress, but as a dynamic balance between anabolic and catabolic forces: the capacity to increase production or imports of vital materials, the interactive benefits of increased interchange, and the ability to both remove and exclude noxious substances and agents, of all descriptions.

Agriculture created the first cities, but hygiene acts as the brake on size.[1] The modern metropolis, with early examples of London, Paris, and New York, were the result of managing both waste streams (principally solid and liquid, thouggh also atmospheric and industrial), public health, and modern policing and crime-fighting.

Public transit haas been essential to supporting flows and densities, with street busses, streetcars, and subways critical to moving the masses of people between factories, offices, retail, education, and other service esstablishments. (Trades, construction, and service work often being excceptions). The de-densified, automobile-based, suburban-sprawl concept has beentried, but results in congestion, smog, trafffic accidents, and vast parking and street overhead (30%+ in most areas).

The key question to me is what the post-covid world's infectious-disease dynamic looks like. Does Covid "just disappear"? Is it one in which SARS-COV-2 itself is endemic, much as rhinovirus, influenza, hepatitis, and HIV are? Is this the first (or only most notably recently successful) of a new age of frequent novel emerging diseases sweeping the globe every few years?[2] Or only an, in retrospect, not especially dangerous pathogen of the sort which seems to sweep the world at 50-100 year intervals or so?[3]

That question hinges on whether disease is a largely exogenous environmental occurrence, or a co-symbiant of human activities and behaviours, co-emerging with them. My view leans toward the latter, based on both evolutionary arguments and historical evidence --- see Kyle Harper's The Fate of Rome for an excellent argument that civilisations and their diseases co-evolve.[4]

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Notes:

1. Almost certainly a generalisable proposition of any networked system. As Tilly and Odlyzko note, network benefits accrue as n * log(n) of nodes, a decreasing though positive factor. If we assume an average network cost funtion of a fixed value per node at any one time --- call it an inconvenience, friction, or hygiene factor, net value becomes:

n * log(n) - k*n

The effective size of a. network is then governed by the costant k.

Which, as noted, is (on average) the same for all nodes (with some good basis for this),but not necessarily constant for the network over time. As hygiene methods and practices improve, k is reduced, but as noxious. vehaviours and phenomena evolve, k my be increased. Thinking or planning that k is fixed in time is a critical error.

2. Looking back, COVID-19 seems to fit a trend of frequent novel respiratory illnesses dating to at least the 1990s: SARS, MERS, H1N1, bird and swine flus. More broadly, AIDS, Zika, hepatitis variants, MRSA, and other infections have become prevalent since the 1970s. Many viral, though bacterial infections, increasingly with antibiotic resistance, are prominant as well.

3. By some measures, Covid-19 is not hugely infectious or deadly, said not to minimise risks, but to note that its enormous enduring global economic impacts could be far worse for a more formidable disease. I'm mindful that full and long-term impacts remain largely unknown. Uncertainty is itself a major risk factor in novel disease.

4. https://press.princeton.edu/books/hardcover/9780691166834/th...