Host factor is a medical

Medicine

Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....

 term referring to the traits of an individual person or animal that affect susceptibility to disease

Disease

A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...

, especially in comparison to other individuals. The term arose in the context of infectious disease

Infectious disease

Infectious diseases, also known as communicable diseases, contagious diseases or transmissible diseases comprise clinically evident illness resulting from the infection, presence and growth of pathogenic biological agents in an individual host organism...

 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
• nutrition
  Nutrition
  Nutrition is the provision, to cells and organisms, of the materials necessary to support life. Many common health problems can be prevented or alleviated with a healthy diet....
  
  al state
• social ties
• previous exposure to the organism or related antigen
  Antigen
  An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...
  
  s
• haplotype
  Haplotype
  A haplotype in genetics is a combination of alleles at adjacent locations on the chromosome that are transmitted together...
  
   or other specific genetic differences of immune function
• substance abuse
  Substance abuse
  A substance-related disorder is an umbrella term used to describe several different conditions associated with several different substances .A substance related disorder is a condition in which an individual uses or abuses a...
  
  
• race

The term is now used in oncology

Oncology

Oncology is a branch of medicine that deals with cancer...

and many other medical contexts related to individual differences of disease vulnerability.