Host factor
Host factor (sometimes known as risk factor) is a medical term referring to the traits of an individual person or animal that affect susceptibility to disease, especially in comparison to other individuals. The term arose in the context of infectious disease research, in contrast to "organism factors", such as the virulence and infectivity of a microbe. Host factors that may vary in a population and affect disease susceptibility can be innate or acquired. Some examples: 
general health
psychological characteristics and attitude
nutritional state
social ties
previous exposure to the organism or related antigens
haplotype or other specific genetic differences of immune function
substance abuse
race

The term is now used in oncology and many other medical contexts related to individual differences of disease vulnerability. See also 
 Vulnerability index
 Epidemiology
 Immunology

References 

Animal diseases
Medical terminology