The change in morphology and behavior of a microglial cell resulting from exposure to a cytokine, chemokine, cellular ligand, or soluble factor. [ http://www.ncbi.nlm.nih.gov/pubmed/9893949 http://www.ncbi.nlm.nih.gov/pubmed/10695728 http://www.ncbi.nlm.nih.gov/pubmed/10626665 http://www.ncbi.nlm.nih.gov/pubmed/12580336 GOC:mgi_curators ]