This study examined how time and handwashing impact the inferential process related to infectious disease transmission among adults and preschoolers. Thirty-eight Japanese adults aged 18–23 years (8 men; 30 women) and 36 Japanese children aged 5–6 years (15 boys; 21 girls) reported their certainty regarding protagonists’ subsequent infection and whether handwashing affects infection risk across three scenarios (same-time, immediately-after, and next-day). Both age groups perceived a significantly higher likelihood of infection in the same-time condition than in the other conditions. Preschoolers estimated a lower likelihood of infection than adults (η _p ² = .25). Thus, preschoolers inferred infectious disease transmission more optimistically than adults. Amidst infectious disease outbreaks, adults must provide guidance regarding preventive measures, while being mindful of preschooler’s propensity to optimistically speculate about contagion.