A network activity detection system is trained to detect network activities of interest such as threats by malicious computer data. The training involves distilling the characteristics of known network activities of interest (e.g., intrusion by computer viruses, exploits, worms, or the like) into a minimal set of meta-expressions. At run-time, the network activity detection system combines the minimal set of meta-expressions with efficient computer algorithms for evaluating meta-expressions to detect known network activities of interest, as well as their unknown variants, among an unknown set of network activity. The network activity detection system may produce appropriate responses upon the detection of network activities of interest.