Timothy is the frailest of the Frisby children, and "when colds or flu or virus infections (come) around he (is) the first to catch them and the slowest to recover" (Chapter 1 - "The Sickness of Timothy Frisbee"). Mrs. Frisby attributes his frailness to an incident that happened to him when "he was only a baby, scarcely bigger than a marble". While playing with the other children, Timothy had wandered away, and had been bitten by something poisonous. When he was later found, "he lay curled in a ball, paralyzed and scarcely able to breathe".
Mr. and Mrs. Frisby immediately carried their tiny son to Mr. Ages, who knew how to make "the draughts and powders that could sometimes save the sick from dying". Mr. Ages examined Timothy, and found "a small red lump near his neck". Mr. Ages determined that it was a spider which had bitten Timothy, "not a black widow, but bad enough". He administered "a milky liquid" to the sick little mouse, holding him upright so that the medicine would "trickle down his throat, for (he) could not swallow". After a short time his muscles unlocked, and he was able to move again. Mr. Ages told the anxious parents that Timothy would be all right, although "he would be weak for a few hours".
Timothy did recover as Mr. Ages had predicted, but "from that time on he tended to stumble a little when he walked, especially when he was tired". He also "never grew as big or as vigorous" as his older brother. Because of these things, Mrs. Frisby believes that the incident with the spider was the cause of Timothy's lifelong frailness (Chapter 2 - "Mr. Ages").