Final answer:
An infectious disease is a condition caused by pathogen invasion and multiplication in the body, which can be spread, often easily, from person to person.
Step-by-step explanation:
An infectious disease is a condition caused by microorganisms such as bacteria, parasites, fungi, viruses, viroids, and prions, that enter and multiply in the body. When infected, these pathogens can lead to disease by either direct invasion of host tissues, production of toxins, or through triggering an immune response that may damage host tissues. Some infectious diseases are communicable, meaning they can be transmitted from person to person either directly or indirectly, and are often also contagious, which indicates they can easily spread between individuals. The degree of contagion often depends on the mode of transmission of the pathogen. For example, measles is highly contagious and can be spread by airborne droplets from coughing or sneezing. In contrast, gonorrhea requires close intimate contact for transmission.