Pathogens can re-emerge during the disturbance of zoonotic reservoirs. These disturbances are becoming more common as previously untouched environments are being developed for new cities. This may have been the source of Zika virus.
Globalisation, especially almost instantaneous air transport, increases the spread of diseases around the world. As demonstrated by the Covid-19 pandemic, it is possible for a pathogen to spread around the world before anyone notices.
Some pathogens are re-emerging due to drug resistance. If it is not possible to treat a bacterial infection, such as gonorrhoea, the number of cases will increase. Having more individuals in higher density cities can lead to increased spread. The globalisation of western lifestyles may also lead to individuals having more sexual partners and further propagate the spread.