<p>Herpes simplex virus utilizes thymidine in the synthesis of deoxyribonucleic acid (DNA), a metabolite necessary for reproduction. Idoxuridine is identical in chemical structure to thymidine except that the 5-methyl group is replaced by iodine. When idoxuridine is substituted for thymidine in DNA, the cell is unable to utilize the DNA and reproduction ceases.</p>