What happens if a category has a zero frequency within a class, and how is this issue commonly addressed (Naive Bayes)?

If a feature appears zero times within a particular class, the computed likelihood score for an observation belonging to that class will be zero, even if the conditional probabilities are much higher for all other features, implying the observation should be assigned to that class. To address this issue, a common practice is to add a small number ? to the frequencies of each feature within each class in order to prevent a zero likelihood score being assigned to any class.