18mo has had temp of 100 or more for 1 1/2 days. Acting normal for most part could temp be do to viral infection?

Most likely. Low grade fever in the summer in this age group is commonly due to viruses. If no productive cough, no foul breath, or other worrisome findings or symptoms, caution is the best thing. Observe, treat fever, hydrate and then if concern, prolonged symptoms or warning signs, get the tot checked out by your pcp.