Infectious diseases are illnesses caused by pathogenic microorganisms such as bacteria, viruses, parasites, or fungi that can be transmitted from one person to another either directly or indirectly (vector-borne). An infection is the infiltration of disease-causing pathogens into an organism's bodily tissues, their multiplication, and the host tissues' reaction to the infectious agents and the toxins they create. An infectious disease, often known as a transmissible or communicable disease, is a condition caused by an infection. Infections are caused by a variety of pathogens, the most common of which being bacteria and viruses. The immune system of hosts can help them fight diseases. Mammalian hosts respond to infections with an innate, generally inflammatory, reaction, which is followed by an adaptive response. Infectious disease refers to the branch of medicine that deals with infections.