Answer:
a. pathogens
Step-by-step explanation:
Infectious diseases are caused by pathogens, which are microorganisms such as bacteria, viruses, fungi, or parasites that can invade the body and cause disease. These pathogens can be transmitted from person to person through various means, such as direct contact, airborne droplets, contaminated food or water, or insect bites. They are capable of multiplying and spreading within the body, leading to infection and the onset of symptoms.