The world was taken by surprise when COVID-19 emerged as a pandemic. Now, mpox, a disease originating from animals, has been labeled a global public health emergency. In a passage from his new book, “Fighting an Invisible Enemy,” virologist Barry Schoub, founding director of South Africa’s National Institute for Communicable Diseases, explores how diseases transfer from animals to humans. Outbreaks and pandemics of new diseases occur in two phases. First, a zoonotic spillover introduces the infectious organism, usually a virus, into humans from animals. The second phase involves various factors that aid in the spread of the pathogen through the human population.
A spillover event happens when a disease agent crosses from animals to humans and establishes itself among people. This can result from humans entering untouched wild animal ecosystems or wildlife moving into or being brought into human areas. Introducing a new animal virus to humans is rare because the virus must find and attach to a specific receptor on the cell surface to initiate infection—a process akin to fitting a key into a lock. When this rare match occurs, the virus begins replicating within host cells and spreads to additional cells. The virus must then spread from one person to another, establishing a chain of transmission that can lead to a disease outbreak. Modern human behavior has significantly contributed to the rise of new communicable diseases.
The global trade, both legal and illegal, in exotic wildlife fosters increasing interactions between wild animals and humans. These interactions can arise from animal trade, agriculture, deforestation, or the consumption of wild animals in places like Far East wet markets. Wildlife might also enter human territories in search of food when displaced from their habitats. Climate change presents a significant threat to human health, causing extreme weather and indirectly affecting human health through habitat destruction and the resulting impact on communicable diseases.
The Nipah virus outbreak in Malaysia and Singapore in 1998/1999 illustrates how environmental destruction and climate change can facilitate new human diseases. This outbreak involved 276 cases, 106 of which were fatal. The fruit bat, the virus’s natural host, faced food shortages due to drought and deforestation-induced haze, which led them to contact domestic pigs that eventually transmitted the virus to humans. The outbreak was controlled by culling a large number of pigs. In ancient times, plagues spread slowly due to limited transportation. Today, modern air travel allows diseases to spread globally within days
Thus, the rise of new communicable diseases in the 21st century is not surprising, as human activities continue to harm the planet’s natural environments, fostering increased interactions between humans and wild animals. These conditions echo the challenges faced by Native Americans in the 15th and 16th centuries, who experienced devastating population losses due to “new” diseases introduced by European colonizers. In these “virgin soil” epidemics, the population faced novel organisms, against which they had no immunity, allowing unimpeded disease spread.