Which of the following is true about virulence?

Virulence refers to the degree of pathogenicity or the ability of a microorganism to cause disease in a host. It is influenced by the specific characteristics of the pathogen itself, such as its ability to invade the host, evade the immune system, and produce toxins.

The statement that it can vary among different strains of pathogens is accurate because different strains of a pathogen can possess varying levels of virulence. For instance, within a single species of bacteria, some strains may cause severe disease while others may cause mild symptoms or none at all. This variability can be attributed to genetic differences, the presence or absence of virulence factors, and adaptations to specific environments.

The other options are less accurate: virulence is not fixed within a host (making the first statement incorrect), it is not directly reduced by antibiotics (as antibiotics target the bacteria rather than altering their virulence), and virulence affects a wide range of organisms, not just humans. Thus, recognizing the variability of virulence among strains is essential in understanding disease dynamics and public health implications.