Zoonosis

A zoonosis is an infectious disease that spreads from animals to humans. These illnesses are caused by a pathogen, which is a tiny infectious agent. Pathogens include viruses, bacteria, parasites, fungi, and prions. When a pathogen jumps from a non-human animal to a person, it creates a zoonotic disease. Interestingly, the reverse can also happen. When humans infect non-humans, the process is called reverse zoonosis or anthroponosis. Understanding these jumps is vital because they shape human health globally.

Pathogens move between species through several specific mechanisms. In a direct zoonosis, the disease passes straight from an animal to a human. This can occur through the air, such as with influenza viruses. It can also happen via bites or contact with saliva, which is how rabies spreads. Some pathogens use a vector, which is a living carrier like a mosquito or a tick. These insects pick up the germ from an animal and carry it to a person. Other diseases are foodborne, meaning they spread through contaminated food or water.

There are many different types of pathogens that cause these diseases. Bacteria like Salmonella or Escherichia coli O157:H7 are common causes of foodborne illness. Viruses can also be the culprit, such as the Ebola virus or the Hepatitis E virus. Hepatitis E is often transmitted through pork products in areas with limited sanitation. Parasites are another major group, including Toxoplasma gondii, which is often found in undercooked meat. Even fungi can be zoonotic, such as those found in pigeon droppings that people might inhale.

Scientists have tracked how these diseases emerge and evolve over time. For example, HIV was a zoonotic disease that moved to humans in the early 20th century. It has since evolved into a disease that exists only in humans. We also see this with influenza. Avian and swine influenza viruses have high zoonotic potential. Sometimes, these animal strains undergo recombination with human flu strains. This process creates new viruses, which can lead to pandemics like the 2009 swine flu.

Zoonotic diseases represent a massive portion of human illness. It is estimated that 60% of all known human infectious diseases are zoonotic. Even more significantly, up to 75% of all emerging infectious diseases are zoonotic in origin. Out of 1,415 known pathogens that infect humans, 61% are zoonotic. These numbers show how closely our health is tied to the animal kingdom. The risk of new diseases appearing is often linked to how many different species live in one area.

Many different environments and activities increase the risk of a spillover event. A spillover occurs when a pathogen moves from an animal reservoir to a human population. Human encroachment into wildlife habitats, such as through deforestation, makes this more likely. Urbanization also plays a role by bringing humans closer to urban wildlife like rodents, bats, and raccoons. Rodents are particularly important reservoirs in cities. They can spread diseases like leptospirosis or hantavirus through bites, scratches, or contact with their urine and droppings.

Human society and industry also influence how these diseases spread. The wildlife trade can increase risk by forcing many different species into small, crowded spaces. This allows pathogens to mix and jump between species more easily. Industrial farming, especially of pigs and chickens, is also considered a primary risk factor. Additionally, climate change alters the distribution of vectors like mosquitoes, changing how diseases move. To fight this, experts suggest a "One Health" approach. This strategy integrates human, animal, and environmental health to manage risks more effectively.