Final answer:
A pathogen is an agent, generally a microorganism, that causes disease. These include viruses, bacteria, fungi, protozoa, and helminths, which the immune system actively defends against.
Step-by-step explanation:
The term for an agent that causes disease in its host is a pathogen. These agents are usually microorganisms, such as viruses, bacteria, fungi, protozoan, and helminths. Pathogens can cause a range of illnesses, from mild to severe, and they possess pathogen-associated molecular patterns (PAMPs) which are unique signatures that our immune system recognizes as foreign. The immune system works tirelessly to defend against these disease-causing agents. Infectious diseases are transported through various vectors and can spread between hosts, posing significant health risks to humans.
Human pathogens include not only viruses and bacteria but also other microorganisms like protozoa and fungi, as well as larger parasites such as helminths. The diseases caused by pathogens can be due to direct tissue damage or the effects of toxins they produce. Fortunately, our immune system has mechanisms to fight off many pathogenic infections, safeguarding our health.