The process of spreading the disease

The process of spreading the disease constitute all the conditions and factors enabling and influencing the transmission of the source of infection (etiological agens, EA), from one organism to antoher – susceptible organism. Contagious agents (metazoa, protozoa, fungi, bacteries, viruses, prions), are the carriers of certain characteristics that allow them to spread:
 * Pathogenicity – the ability of EA to induce a specific pathological condition in the organism.
 * Virulence – degree of pathogenicity of individual EA strains (variable depending on toxicity and invasiveness).
 * Toxicity – the ability of EA to damage the organism with produced toxins.
 * Invasiveness – the ability to penetrate, persist, multiply or undergo a certain degree of development in the host's organism.

A certain amount of EA always enters the body – infectious dose. The originator may have the ability to resist physical effects (different temperature, radiation, drying) – resistatance, usually it has the ability to multiply and to infect an intermediate host or vector.

To prevent the spread of the disease, the epidemiological chain must be broken: We can even
 * EA source → spread (path of transmission) EA → susceptible organism.
 * to get rid of the source by isolation, or destroying, or cure, so the EA couldn't spread any further;
 * prevent transmission to susceptible individuals in the external environment after expulsion of EA by the infected individual (by methods disinfection, disinsection);
 * or to change the individual's susceptibility to EA (immunisation, chemoprofylaxis, fagoprofylaxis).

Focus of infection can be human or animal.

Susceptible organism is a human, who lacks any type of immunity or resistance against a certain pathogenical agens, which could prevent infection after exposure.