Why are some viral diseases shared between species and others not?

The ability of a virus to infect multiple species, known as cross-species transmission, or "zoonotic transmission", depends on several factors. Let's compare two scenarios where certain viral diseases are shared between species and others are not:

Shared Between Species (eg: Influenza virus):

1. Viral Genome: The genetic makeup of the virus plays a crucial role. If a virus can recognize and bind to cellular receptors on different host species, it can potentially infect them. Mutations or sequence variations in the viral genome may broaden its host range.

2. Host Specificity: Some viruses have a relatively wide host range, meaning they can infect multiple species. For example, many strains of the influenza virus can infect both humans and animals, leading to shared diseases like the swine flu.

3. Environmental Factors: Environmental factors can influence the spread of viruses across species. Close proximity between different species, such as humans and livestock, can increase the opportunities for cross-species transmission.

4. Evolution: Over time, viruses may evolve to adapt to new hosts. Some animal viruses can undergo mutations that allow them to infect humans, leading to the emergence of new zoonoses.

Not Shared Between Species:

1. Species-Specific Receptors: Some viruses rely on specific cellular receptors that are only present on particular host species. If a virus lacks the matching receptors on another species, infection is unlikely.

2. Immune Response: Different species have variations in their immune systems, which can prevent certain viral infections. An immune response that is effective against a virus in one species may not be as efficient in another species.

3. Cross-Species Barriers: Certain biological barriers, such as species-specific physiological or biochemical differences, can limit the ability of a virus to establish successful infection in a new host.

4. Transmission Constraints: Some viruses may be highly transmissible within a single species but have difficulty transmitting efficiently to other species due to factors like species-specific behaviors or transmission routes.

In summary, the ability of viral diseases to be shared between species is determined by the combination of viral characteristics, host factors, environmental conditions, and evolutionary dynamics. Some viruses have the potential to cross-infect multiple species and cause zoonotic diseases, while others are more confined to specific host ranges.