In up to 20 percent of the world’s most linked cities, new infectious diseases could spread undetected, which are “slap bang in the middle” of high-risk spillover zones but lack the health infrastructure needed to contain dangerous new pathogens. Researchers also found that 40 percent of the cities that are major international transport hubs are within 50 km of regions with rich biodiversity and a large mix of animals and humans, offering enough opportunities for infectious diseases to spread to humans. New diseases could remain undetected as the pathogens have recently emerged within a population or those that are increasingly growing or threatening to increase the incidence or geographic range in the immediate future. Infections that are unknown can be caused by:

• Previously unknown or undetected infectious agents
• Established agents who have spread to new geographic places or to new populations
• Previously recognized agents whose function has previously gone unrecognized in specific diseases.
• Re-emergence of agents whose disease incidence has been greatly reduced in the past, but whose disease incidence has re-emerged. This class of diseases is classified as infectious diseases that are re-emerging.

In the introduction of new pathogens or the re-emergence of “old” infectious diseases, there are several variables involved. Some are the product of natural processes, such as the production over time of pathogens, but many are the result of human actions and practices. Consider how, especially in the last century, the relationship between the human population and our environment has changed. Population growth, migration from rural areas to cities, international air travel, poverty, conflicts, and disruptive ecological changes due to economic development and land use are factors that have led to these changes.

As a factor in the emergence of new diseases, climate change is rapidly becoming a concern. Diseases will expand into new geographic areas as the Earth’s atmosphere warms and ecosystems are altered. Warming temperatures, for example, cause mosquitoes – and the diseases they spread – to extend their range to regions where they have not been detected before. Antimicrobial resistance – the acquired resistance of pathogens to antimicrobial drugs such as antibiotics – is another factor that is particularly important in the re-emergence of illnesses. Over time, bacteria, viruses, and other microorganisms may alter and develop resistance to medicines used to treat diseases caused by the pathogen.

In order to mitigate potential risks, Covid-19 is a wake-up call for greater investment in health infrastructure and airport biosecurity, thus prioritizing sustainable development to reduce human invasion of highly biodiverse areas where hazardous pathogens can occur. Because global pandemics aren’t once in 100-year events.