0.01ml is extremely low dosage for an infection of any kind. the norm is at least 0.02ml TWICE a day, depending on the weight of the rat.
tumors are more common in older rats, but can occur at any age, more likely if they have bad breeding, ie from a petshop = rodent farm usually.

an infection of what? urinary tract?
pyometra is also a possiblity, of which it can be life threatening if not treated correctly.