According to evolutionary medicine, optimal virulence increases with horizontal transmission (between non-relatives) and decreases with vertical transmission (from parent to child). This is because the fitness of the host is bound to the fitness in vertical transmission but is not so bound in horizontal transmission.

What makes a pathogen more virulent?

In the previous section, we explained that some pathogens are more virulent than others. This is due to the unique virulence factors produced by individual pathogens, which determine the extent and severity of disease they may cause.

What causes virulence factor?

Virulence factors are bacteria-associated molecules that are required for a bacterium to cause disease while infecting eukaryotic hosts such as humans. A surprisingly large number of virulence factors are encoded by prophage infecting bacterial pathogens, such as cholera toxin, Shiga toxin, and diphtheria toxin.