With regards to the host, the integrity and preparedness of the immune system are critical. Some animals are born with defects of the immune system which make them unable to combat infectious diseases. Age of exposure is an important host factor since young animals are almost always more susceptible than older ones. In addition, poor nutrition can adversely influence an animal's resistance as can the presence of a concurrent illness or parasite infestation. The nature of the environment can also have a profound effect on the outcome of a disease process.
Population density is also important since overcrowding almost invariably leads to disease problems.
With infectious diseases in particular, the interchange of populations of animals is apt to be troublesome. Many cases of infectious disease outbreaks can be traced to the introduction of new animals into a herd. Such animals may appear healthy but may be incubating a disease or may be carriers of microorganisms to which the main herd has not been exposed. Of course, transfer of infection can also occur from the herd to the new animals. Finally, there are factors associated with the infectious microorganisms themselves which can influence the nature of the disease produced.