This is the row index into the table of the highest all-time virus detections on this gateway, as opposed to the current-period table tracked elsewhere in this MIB. An admin uses this longer-horizon table to spot which threat has been the single most persistent problem on this specific gateway over its whole operational history, useful context when deciding where to invest in additional endpoint controls. For example, index 1 in this table representing an older but historically dominant threat, different from whatever currently tops the top viruses right now table.