Virulence factors are the specific molecular tools, toxins, adhesion molecules that let a microbe stick to host tissue, and enzymes that help it invade or evade the immune system, that actually let a pathogenic microbe cause disease in a host, rather than simply being present without causing harm. Many disease-causing bacteria don't deploy their full set of virulence factors from the moment they first infect a host; instead, they use a coordinated system called quorum sensing, only switching key virulence factors on once the local bacterial population's density crosses a specific chemical-signal threshold.
Virulence factors are what separate a harmful infection from a harmless presence
Simply being present in or on a host isn't, by itself, what makes a microbe pathogenic — plenty of bacteria live on or inside a host without causing any disease at all. What makes a microbe capable of actually causing disease is its possession of specific virulence factors: molecular tools that let it adhere firmly to host tissue rather than being easily cleared, invade past physical barriers, evade or suppress the host's immune response, and, in many cases, produce toxins that directly damage host cells or tissue. A microbe's overall virulence, how severe a disease it's capable of causing, depends heavily on which of these specific tools it actually has and how effectively it deploys them.
Quorum sensing lets a bacterial population wait until it has the numbers
Many disease-causing bacteria don't express their most damaging virulence factors constantly from the moment of infection; instead they use quorum sensing, a system in which individual bacterial cells release and detect a chemical signalling molecule, allowing the population as a whole to effectively gauge its own local density. Only once that chemical signal reaches a high enough concentration, indicating the population has grown large enough to mount an effective, coordinated attack rather than being easily cleared by the host's immune system while still small, do the bacteria switch on their full complement of virulence factors together. This coordinated delay lets the bacterial population avoid provoking a strong immune response prematurely, while it's still too small a population to survive that response.
What we're still unsure about
The basic categories of bacterial virulence factors, and quorum sensing's role in coordinating when many bacterial species switch them on, are well established microbiological knowledge, confirmed across a wide range of studied pathogens. What remains more genuinely an active area of ongoing research is how to therapeutically exploit quorum sensing itself, since disrupting a pathogen's ability to sense its own population density, rather than trying to kill the bacteria outright the way conventional antibiotics do, is being actively explored as a way to blunt an infection's severity without necessarily driving the same kind of resistance that killing-based antibiotics tend to select for — researchers are still working out how broadly effective and how durable that quorum-sensing-disruption approach actually turns out to be across different pathogens, rather than it being an established clinical treatment yet.
This sits inside Pathogenicity & Virulence Factors, one of seven topics in Microbiology, one of six domains in Biology, one of seventeen subjects the app can quiz you on.