The present invention relates to a method of treating infectious diseases, that involves: (a) providing an alpha-glycosidically linked starch polysaccharide derivative; and (b) inhibiting the growth of an infectious disease by administering a composition comprising the alpha-glycosidically linked starch polysaccharide derivative. The alpha-glycosidically linked starch polysaccharide derivative represented by the following general formula I, in which: the alpha-glycosidically linked starch polysaccharide derivative has a degree of quaternary ammonium group substitution of from 0.4 to 2.0; n is 2-4; R1 is selected from the group consisting of C1-4 alkyl, benzyl and benzyl substituted with a member selected from the group consisting of C1-3 alkyl, halogen, alcoxy, carbamoyl, alkoxycarbonyl, cyano, dialkylamino and hydrogen; R2 and R3 are each independently selected from the group consisting of C1-4 alkyl, benzyl and benzyl substituted with a member selected from the group consisting of C1-3 alkyl, halogen, alcoxy, carbamoyl, alkoxycarbonyl, cyano and dialkylamino; and X is an anion selected from the group consisting of halide, hydroxide, sulfate, hydrogen sulfate and carboxylate.