1. The beginning or incursion of a disease. 2. Local spread of a malignant neoplasm by infiltration or destruction of adjacent tissue; for epithelial neoplasms, i. signifies infiltration beneath the epithelial basement membrane. 3. Entrance of foreign cells into a tissue, such as polymorphonuclear leukocytes in inflammation. [L. invasio, fr. in-vado, pp. -vasus, to go into, attack]
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a) the penetration of the body of a host by a microorganism
b) the spread and multiplication of a pathogenic microorganism or of malignant cells in the body of a host
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n.
the spread of cancer into neighbouring normal structures; it is one of the cardinal features of malignancy.
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in·va·sion (in-vaґzhən) [L. invasio, from in into + vadere to go] 1. the attack or onset of a disease. 2. the infiltration and active destruction of surrounding tissue by a malignant tumor.Medical dictionary. 2011.