Zika virus is carried by the Aedes aegypti mosquito throughout most of the world’s tropical regions. In pregnant women, it is known to cause infants to be born with microcephaly and other complications. The most recent Zika epidemic of note was the 2015-16 outbreak in Brazil, which eventually spread across the Americas.

Many environmental factors have played a role in the spread of Zika virus, including climate change, deforestation, and more. As the global climate grows warmer, more and more areas have started experiencing tropical weather suitable for Zika-carrying mosquitoes. Deforestation for agriculture disrupts streams and rivers, creating pools of warm water perfect for mosquito breeding. These effects are compounded by the rise in global travel and migration, which has allowed Zika virus to spread across the world at exponential rates.

Nipah virus is a bat-borne disease associated with encephalitic symptoms (i.e. brain swelling), with an average mortality rate of 75%. Ever since its first appearance in 1999 in Malaysia, outbreaks have occurred periodically throughout Southeast Asia. According to the World Health Organization, it is one of the 10 most important pathogens with pandemic potential to monitor.

Nipah virus is endemic to fruit bats, and its outbreaks have all stemmed from exposure to bat secretions. In the 1999 outbreak, Malaysia’s rapid economic growth was driving an increase in commercial farms; these in turn led to large-scale deforestation to clear agricultural land. Such proximity to the habitats of wild fruit bats made it easy for Nipah-infected secretions like bat urine or saliva to make their way into the farms’ pigpens, where they infected pigs, then farmers. Later outbreaks in Bangladesh and India had similar agricultural origins, caused by consumption of fruit products contaminated with secretions from infected fruit bats.

Influenza has been responsible for a number of historic pandemics, including the Spanish flu and the 2009 H1N1 pandemic. Strains of the virus occur in many farmed species like pigs and poultry, from which they can spread to farm workers and become highly contagious. The particular challenge of influenza lies in its unpredictability: a number of strains are currently endemic in agricultural pig and poultry populations across the world, and may spill over into humans at any point.

H5N1 (bird flu) is a particular strain of Influenza A present in poultry that is currently considered to have major pandemic potential. It does not currently transmit between humans very effectively; however, it is highly lethal in chickens and has been shown to easily mutate into a human-transmissible strain.

Rabies is an animal-transmitted virus with the highest case fatality of any known infectious agent. It is most commonly spread through dog bites, and is present across all continents. Rabies is a prime example of the need for a One Health approach: human rabies is only effectively prevented by targeting the disease in animals. Most rabies control programs utilize mass vaccination and neutering of at-risk animals, as well as mass awareness and case reporting programs. However, most current rabies programs do not adequately address rabies cases outside of domestic or community animals; this requires greater rabies surveillance among wildlife populations.

Cholera is an infectious disease that is spread from food or water contaminated with the bacterium Vibrio cholera, which causes severe, often lethal diarrhea. Cholera has been around for centuries, but has been largely mitigated by modern sanitation measures; however, it continues to affect an estimated 3 to 5 million people worldwide each year.

Cholera is spread when there is a lack of healthcare facilities, proper sanitation, and waste management. In recent years, climate change-induced environmental factors like increased rainfall, flooding, and winds have contributed to further contamination of water sources. The Vibrio bacteria can also survive on the external surfaces of insects, which have the potential to contaminate food directly. The ongoing cholera crisis in Yemen -- the worst of modern times -- has also been exacerbated by war and poverty, with crippled sanitation and healthcare facilities that prevent people from accessing clean water or oral vaccines.