The response from immune system varies and its ability to respond is influenced by these factors.
- The virulence of the infectious agent.
Some strains of infectious agents are more likely to overwhelm the immune system than others.
- The dose of infectious agent.
Generally speaking, the higher the number of infectious particles a pet is exposed to the more likely it is that the immune system will be overwhelmed.
- The presence of other illness.
Some infections, for example distemper in dogs and infectious enteritis in cats depress the immune system but any co-existing illness or infection drains the body of essential nutrients, ties up parts of the immune system and reduces its ability to respond efficiently.
- Poor nutrition.
Antibodies are proteins and good nutrition provides the amino acid building blocks for protective antibodies. Antioxidants such as selenium, zinc and the vitamins A, E and C may also play an as yet poorly understood role in maintaining an efficient immune system.
- Environmental conditions.
Changes in environmental temperature and humidity somehow affect the efficiency of the immune system. So too does social stress, certainly in cats, although exactly how this happens is one of the great remaining questions in immunology.