The global rise of antimicrobial resistance and the limited development of new anti-infective agents have renewed interest in alternative and complementary therapeutic strategies, including herbal medicines. This review highlights the potential role of medicinal plants in the prevention and treatment of infectious diseases, with a focus on their antimicrobial, antiviral, and immunomodulatory properties. Numerous plant-derived compounds, including alkaloids, flavonoids, terpenoids, and phenolic substances, have demonstrated inhibitory activity against a wide range of pathogens in preclinical studies. These effects are mediated through multiple mechanisms such as disruption of microbial cell membranes, inhibition of essential enzymes, and modulation of host immune responses. Additionally, several medicinal plants, including Allium sativum, Curcuma longa, and Thymus vulgaris, have shown promising activity against clinically relevant bacterial and viral pathogens. Despite these encouraging findings, the clinical translation of herbal therapies remains limited due to a lack of standardized formulations, variability in bioactive constituents, and insufficient high-quality randomized clinical trials. Safety concerns, including potential herb–drug interactions and organ toxicity, further underscore the need for cautious clinical application. Overall, herbal medicines may serve as valuable adjuncts rather than replacements for conventional antimicrobial therapy. Future research should prioritize rigorous clinical evaluation, standardization of herbal preparations, and mechanistic studies to better define their role in evidence-based infectious disease management.