pernicious
/pər-ˈni-shəs/
adjective
highly injurious or destructive : deadly
wicked
pernicious baneful noxious deleterious detrimental mean exceedingly harmful. pernicious implies irreparable harm done through evil or insidious corrupting or undermining. baneful implies injury through poisoning or destroying.
noun
a severe megaloblastic anemia that is marked by a progressive decrease in the number of red blood cells and by pallor, weakness, and gastrointestinal and nervous disturbances and is caused by malabsorption of vitamin B12 due to the absence of intrinsic factor