A pathogen is a bacterium, virus, or some other microorganism that is capable of causing disease, and that capability is called pathogenicity. Every pathogen needs to do five things to be successful, colonize the host, find a nutritionally compatible niche in the host body, avoid, subvert, or circumvent the host immune system, replicate using the host's resources, and exit the host to spread to a new host. Virulence refers both to the pathogen's ability to infect a resistant host and the degree of damage it can cause. To determine a quantifiable value for a pathogen's virulence, we have to look at whether or not it does one or more of five specific things, and if so, how well. The most virulent of pathogens will do most or all of these very well, while less virulent will accomplish fewer.
Evade the Immune System
In order to set up a long term infection, a pathogen must either evade or defeat the host's two immune systems. The innate system is comprised of a variety of non-specific immune cells and proteins that act as the body's first responders. They arrive first on the site of infection, and are capable of destroying most of the less virulent pathogens. Those that do manage to evade or defeat the innate system must then face the adaptive immune system, which consists of antibodies and lymphocytes specifically programmed to fight the current pathogen.
With humans and commercial or pet animals, vaccines and antibiotics can often be used to boost the immune system and help fight off a pathogen. The human immune system is very good at fighting off most common pathogens by itself. Even when a pathogen makes enough trouble for a person to actually feel sick, there is a good chance the body will eventually defeat the pathogen. This is why people usually eventually get over colds and gastroenteritis (stomach “flu”), whether they're treated or not. Even when that is not the case, medications and treatments for both the symptoms and the pathogen itself can often help the immune system knock out the infection. Some of the more virulent pathogens, however, have become resistant to treatment, like the many antibiotic resistant strains of bacteria that have recently evolved. Others, like HIV, actually attack the immune system directly.
Grow and Reproduce Rapidly
The faster a pathogen reproduces, the farther it can spread, both in the single host and from host to host, in a short time. A rapid growth rate increases the effect of any other virulence factors present, and will require more aggressive treatment. A slowly growing pathogen, on the other hand, might still have some very unhealthy effects on its host, but as it moves slower, the host has more time to fight it off.
Pathogens that grow and reproduce rapidly are problematic for another reasons as well. As a pathogen reproduces, random mutation and exposure to the host environment and any attempt at treatment can cause it to evolve to be more resistant to the host immune system and any previously encountered treatments. The faster this reproduction occurs, the faster the strain as a whole evolves. A rapidly growing pathogen is more likely to resist the immune system and develop antibacterial resistances as well.
Many pathogens produce toxins that have a variety of negative effects on host cells. These toxins promote disease by directly harming the host's tissue and by disabling the immune system. Some of these toxins target and destroy a specific part of the cell, while other toxins interfere with the cell's internal processes. Cholera and E. coli both produce toxins that increase the intracellular cAMP levels and cause secretion of fluids and electrolytes in the intestines, causing diarrhea. Tetanus releases a toxin that inhibits neurotransmitter release from inhibitory neurons in the central nervous system and causes spastic paralysis. Many more pathogens, such as pseudomonas, produce toxins that actually destroy the host cell, whether by attacking a specific part or by inhibiting the production of proteins.
Destroy Host Cells
The most virulent of pathogens actually destroy their host cells. Some do this by the production of toxins. Others, however, simply feed off the cell or rupture the cell while reproducing. This is far less damaging than it may initially seem in pathogens that grow and reproduce very slowly and are easily defeated by the immune system or by medical treatment. However, for rapidly growing, rapidly reproducing pathogens that easily resist the immune system and medical treatment, this can be quite devastating, and can often lead to the death of the host organism.
Interfering with Cell Functions
Pathogens exploit several eukaryotic signaling pathways during infection. The more virulent pathogens have evolved specific effectors and toxins to hijack host cell machinery for their own benefit. In doing this, they subvert the cell's normal functions. Pathogens can use the host cell's own internal processes to replicate and care for the pathogen instead of itself. Pathogens can make host cells hide them from the immune system. They can even cause programmed cell death to occur quickly, to avoid destruction of the pathogen, or slowly, to grant the pathogen more time for replication.
Pathogens are microorganisms, such as bacteria and viruses, that cause disease in a host organism. Virulence is the quantifiable measure of a host's ability to infect and damage a host. The more virulent the pathogen, the more likely it is to successfully evade the host's immune system, grow and reproduce rapidly within the host, and damage the host by excreting toxins, interfering with normal cell function, and killing host cells.