-the t)l"'(':u:-n,·,· or nh:-. cncc of whkh mny,;crvl' ns llli111ulu11 tn lnilhll<' or purp ·tuttlc 11 disc:1sc pro,(-..... 111is lrnppc11,; onlywl Hn tht. ' 11grn11 cornc11 in contn<:t with n r;u,iccplil. ile host :111d unolt>r prtlp<. 'r environm ental r1111rlitio11. s. Acnt E11arnpte e1olog1cal vlnn, bactl!f'fa, fungu,. para~ile Chrrn1cal !@ad, mt'rcurv. ln~«ttclde Physical hurnldtty, atfl\osph11rlc pr OY11re. rlldlatton MC!Chanical stab, trauma Nutritive lr Ofl or Iodine deficiency, cholesterol A host is any organism that harbors uml provides 11ourishrn cnt for onothcr ori. ani:m1. TI1e characteristics of I he host will affect l,is or it. s risk of cxp Mnrc 10 sources of infection and his or its s11sccptihility or resistnncc. The rcsistt111l·c 11f lhc host may be specific or non-specific. Specific rc!'<istancc re. ~111ts fr111n 1111 i111mu11o ln1,:ic experience such as undergoing immuni1. a tion or vacdnnticm. Nun~p1. :dlic r(·~istanc:c re.. sults from an intact skin, muco,1s membrane, reflexes ns lacrimalion. coughing, diarrhea. or vomiting. They can be maintain ed U1rough 1:,ersonal hygienic prnctices, environmental sanitalioi;t, proper nutrition and n healthy lifestyle. Since the nurse considers the community as /\ host she protects tbe h Mhh of the oommu. nity by increasing its herd immunit} '· Herd imrnunil y is the probability of a group or community developi ng an epidemic upon introduc tion of an infectious agent. It is the proportio n of the immunes and the susceptible in Lhc group. The environme nt is the sum total of all e:..1:ernal conditions and innuences that affect the life and developme nt of an organism. The environm ent both affects the agent and the host. There are three components of the environm ent: 1. Physical environment is composed of the inanimate surroundings such as the geophysical conditions or tbe climate; 2. Biological environment makes up the lhing things around us such as plant and animal life; 3. Socio-economic environment which maybe in the form oflevel of economic developmen t of the community, presence of social disruptions and the like. The three elements of the ecologic triad interact with one another in an attempt to maintain equilibrium. Any major change in any one of the factors may bring about a disturbanc e in the equilibrium provoking the appearance of a health problem. Natural H;story of Disease Describing the natural history of disease seeks to identify factors related lo the course of a disease once established in order to determine its duration and the probability 187 