What is the definition of a 'pathogen' in medical terms?

A pathogen is defined as a microbe that is harmful and capable of causing disease. This includes a variety of microorganisms such as bacteria, viruses, fungi, and parasites that can infect a host and lead to illness. The key characteristic of a pathogen is its ability to disrupt normal biological functions and provoke an immune response, often resulting in symptomatic disease.

The other definitions provided do not align with the established meaning of a pathogen. Beneficial microbes refer to those that have a positive effect on health, such as probiotics, while allergies are immune responses to typically harmless substances, not caused by pathogens. The fourth option describes a condition specifically linked to viruses, which is too narrow and does not encompass all types of pathogens. This understanding is crucial for recognizing the importance of infection control and treatment strategies in the medical field.