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It is currently thought that its spread may be limited due to a lack of susceptible host trees surrounding its current distribution . We can also attempt to eliminate " living bridges " of susceptible host materials that can act as stepping stones between populations , or plant strips of resistant plants ( or non-host plants ) to act as barriers between adjacent susceptible host populations . The reason for this is that the tissue form of anthrax bacterium , on coming into contact with the air , changes to the environmentally resistant spores , which then serve as the means for re-infection of susceptible hosts . Outbreaks of the redheaded pine sawfly can usually be minimized by avoiding the planting of susceptible hosts where competition for moisture and nutrients may be great or where soil conditions are marginal for hard pine growth . Localized resurgence is more likely due to the following : ( 1 ) a decrease in vigilance in monitoring and sanitation , ( 2 ) a build-up in populations of the insect vectors , or ( 3 ) ingrowth of susceptible host trees in the wild . The warmth allows the pathogens to multiply their range and infect populations of susceptible hosts . LIFE HISTORY : Females bore into twigs , branches , or small trunks of susceptible hosts . Factors to be considered in permitting use of exotic viruses included the presence or absence of susceptible hosts ( of viruses , vectors or both ) both in the field and in greenhouse cultivation , the importance and intensity of commercial cultivation of susceptible crops in the area , and the ability of the vector to survive under adverse climatic conditions . The resulting diarrhea enables shedding of viable transmissible H.pylori providing the opportunity for new infection of susceptible hosts . These generally rely on decreasing exposure and increasing immunity of susceptible hosts .