Zoonosis is :

Question

Zoonosis is :

Accepted Answer

The correct option is 2: Diseases of animals that can be transmitted to humans.

Zoonosis, derived from the Greek words *zoon* (animal) and *nosos* (sickness), is a fundamental concept in both veterinary medicine and public health. According to international health organizations like the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC), a zoonosis (plural: zoonoses) is defined as any disease or infection that is naturally transmissible from vertebrate animals to humans. These diseases are the result of pathogens—including viruses, bacteria, parasites, and fungi—that exist in an animal reservoir population and can jump the species barrier to infect a human host. The core concept is the direction of transmission: *animal to human*. This critical transfer of infectious agents accounts for the majority of emerging infectious diseases globally, making the study and control of zoonoses essential for preventing future pandemics. Classic examples include Rabies (transmitted through bites), Lyme disease (transmitted via ticks from rodents), and the recent coronaviruses. Therefore, the most accurate and encompassing definition provided is that which describes the transfer of disease from an animal source to a human.

The incorrect option is 1: Diseases of human that can be transmitted to animals.
This statement describes the reverse situation, which is formally termed **anthroponosis** or, more specifically, **reverse zoonosis** (or *zooanthroponosis*). Anthroponoses are diseases that originate in humans and can then be transmitted to animals, effectively moving in the opposite direction of a zoonosis. While such transmission events certainly occur—for example, humans transmitting *Mycobacterium tuberculosis* to elephants or dogs catching the flu from their owners—they do not fit the established definition of a zoonosis, which focuses on the primary threat posed to humans from animal reservoirs. The distinction in transmission direction is critical for epidemiological classification.

The incorrect option is 3: A group of gram +ve bacteria infecting animals.
This option incorrectly attempts to define zoonosis as a specific group of pathogens. Zoonoses are defined by their **mode of transmission** and the **species barrier they cross**, not by the type of microorganism that causes them. Gram-positive bacteria, such as *Staphylococcus* species, are a type of pathogen, but they are only one of many categories of infectious agents (viruses, Gram-negative bacteria, protozoa, fungi, prions) that can cause a zoonotic disease. Therefore, limiting the definition of zoonosis to a particular bacterial staining characteristic is factually wrong and conceptually incomplete for a complete understanding of the term.

The incorrect option is 4: A group of gram -ve bacteria infecting animals.
Similar to the previous option, this choice incorrectly narrows the definition of zoonosis to a specific microbiological classification. Gram-negative bacteria, like *Salmonella* or *Escherichia coli* O157:H7, can indeed cause significant zoonotic diseases that are transmitted from animals to humans. However, the term Zoonosis is an epidemiological classification that describes the relationship between the host species. Since a zoonotic disease can be caused by any major class of pathogen, whether viral (e.g., SARS-CoV-2), parasitic (e.g., Toxoplasmosis), or bacterial (regardless of Gram stain), this definition is too restrictive and thus fundamentally incorrect.