Here's a deeper look into it. Size and structure of the virus: It is the structure of the virus that causes all the difference. “All coronaviruses are just about the same size.
It is certainly the structure, but a virus has several adaptations that make it more likely to be passed from one person to another," said Dr Keith Roach, Associate Professor of Clinical Medicine, Weill-Cornell; syndicated columnist To Your Good Health, as quoted by Sun Journal.
Viruses that can reproduce faster and produce a high number of viral particles are more likely to be transmitted from one person to another.
Furthermore, viruses that have more spike proteins, which attach to human cells and facilitate viral entry, are also more likely to spread. Immune evasive properties: Another important factor is the virus's ability to evade the immune system.