Pathogens differ in their virulence, contagiousness, and their modes of transmission (Table 1). These differences exist not
only between pathogens, but for virulence and contagiousness, can also differ between strains of the same species of pathogen.
A more virulent pathogen causes more severe clinical signs of disease and a greater likelihood of death following infection.
A pathogen with greater contagiousness will infect a larger number of animals in a shorter period of time when introduced
into a population. These factors are not related, in that a highly virulent pathogen may not be very contagious and a very
contagious agent may not be highly virulent. In addition, different pathogens have various methods of transmission that impact
how they interact with a host population. Some pathogens are spread via inhalation or ingestion. Infectious agents spread
in these ways are further differentiated by the length of time the agent can survive outside the host in the environment,
by the distance they can travel and still be infectious, and by the age of host that is susceptible to infection or disease.
Other pathogens are spread only through sexual contact and are not contagious outside the act of mating. And still other pathogens
require an intermediate host or transmitting fomite such as an insect, snail, or other mammal.
Table 1. Examples of pathogen factors affecting diseases of cattle
A cattle population's environment includes its housing type, animal density, air quality, weather effects, mud, dust, footing,
and health antagonists such as internal parasite burden, external parasite burden, and social stress. These environmental
factors influence the innate immunity of a herd by their impact on immunosuppression.
In addition to effects on immunosuppression, a herd's environment also dictates the "animal flow" or contact and mixing patterns
of potentially infectious and susceptible animals. Some infectious agents preferentially infect only certain ages of cattle.
Mycobacterium avim subspecies paratuberculosis (Johne's) is primarily infectious to young animals, while Trichomonas fetus (Trichomoniasis) is primarily infectious to older bulls. Some infectious agents will infect all ages of cattle, but are only
likely to cause disease in certain ages. Rotavirus and coronavirus infections are likely to cause clinical disease (calf scours)
in young calves, but not in adults. In contrast, initial infection with the parasite Anaplasma marginale (anaplasmosis) is not likely to cause clinical disease in young animals but will cause disease in adult animals.
The two primary animal factors that affect protection of cattle herds from disease are specific and innate immunity. Specific
immunity relates to an immune response directed at a specific infectious agent that the animal has been exposed to in the
past, either via natural infection or vaccination, for which "memory" remains. Innate immunity is strongly influenced by the
overall health of the animal. Nutritional status such as adequate energy, protein, and dietary requirements of vitamins and
minerals impacts an animal's overall health and immune status. Stress due to crowding, inclement weather, unsanitary housing,
or concurrent disease can cause varying levels of immune suppression.
In populations of animals, not only do pathogen factors such as virulence, and the length of the latent and infectious periods
influence the number of animals infected; but animal factors such as the number of immunologically protected individuals (either
due to specific or innate immunity) also determine the number of individuals the pathogen is able to infect and the speed
of spread through a population.