What Causes Fever In Infant?

3 year old woke up with a fever on Monday noticed white on throat very red. Took him to dr and rapid strep was negative! Thursday he woke up with a 102.6 fever took him back to dr they tested for the flu that was also negative! It's day 5 and he still has a fever that I am treating with motrin no other signs of a cold. how many days should a toddler have a fever if it's just a viral infection?