What defines a drug with a "narrow spectrum" of activity?

A drug with a "narrow spectrum" of activity is defined by its effectiveness against a limited array of microbial types. This means that such a drug is typically designed to target specific pathogens, which is particularly beneficial in treating infections caused by those organisms while minimizing the impact on others.

For example, a narrow-spectrum antibiotic may effectively combat a particular strain of bacteria but not exert its effects on a wide variety of different bacteria or other types of microorganisms. This targeted approach can reduce the risk of disrupting the body’s normal flora and helps to prevent the development of resistance among a broader range of organisms. Such specificity is valuable in certain clinical scenarios where the causative agent of an infection is known, allowing for tailored treatment that minimizes collateral damage to other beneficial microbes.

In contrast, drugs that are effective against all types of microorganisms would be classified as broad-spectrum, and drugs that only affect viral pathogens would be specific to viral infections, which does not characterize the narrow spectrum definition. Additionally, targeting both fungal and bacterial infections simultaneously suggests a broader spectrum of action, rather than the narrow focus described in the correct answer.