Viral infection is a complicated dynamic process, in which viruses intrude into cells to duplicate themselves and trigger succeeding biological processes regulated by genes. It may lead to a serious disaster to human’s health. A scheme is proposed to monitor the response of cells after being infected by viruses. Co-expression levels of genes measured at successive time points form a gene expression profile sequence, which is mapped to a temporal gene regulatory network. The fission and fusion of the communities of the networks are used to find the active parts. We investigated an experiment of injection of flu viruses into a total of 17 healthy volunteers, which develop into an infected group and a survival group. The survival group is much more chaotic, i.e. there occur complicated fissions and fusions of communities over the whole network. For the infected group, the most active part of the regulatory network forms a single community, but it is included in one of the large communities and completely conservative in the survival group. There are a total of six and seven genes in the active structure that take part in the Parkinson’s disease and the ribosome pathways, respectively. Actually, a total of 30 genes (covering [Formula: see text]) of the genes in the active structure participate in the neuro-degeneration and its related pathways. This scheme can be extended straightforwardly to extract characteristics of trajectories of complex systems.