A virus and its host cell have a similar word pattern (k-tuple).The scores of the DNA sequences of the virus and its host cell,which are obtained by means of the word pattern,are often higher than those of random host cells,that is to say,the DNA sequence of the virus is similar to that of its host.On the basis of this principle,two alignment-free statistics D^S₂ and D^*₂ are adopted to acquire the scores between the DNA sequence of the virus and that of its host cell in this paper.Then,the scores are compared with the threshold，so as to judge whether the vi- rus can infect the host cell.Experimental results show that,when k =5 (k is the size of k-tuple) and Markov order is 1,both of the statistics and can describe the similarity between the virus and its host cell in genes，and that the optimal threshold of D^S₂ and D^*₂ from the ROC (Receiver Operator Characteristic) curves can be used to judge whether the virus can infect the host cell.