Infectious diseases are caused by pathogenic microorganisms, such as bacteria, viruses, parasites or fungi; the diseases can be spread, directly or indirectly, from one person to another. Zoonotic diseases are infectious diseases of animals that can cause disease when transmitted to humans.
Infectious diseases, also known as
transmissible diseases or communicable diseases comprise clinically evident
illness (i.e., characteristic medical signs and/or symptoms of disease) resulting
from the infection, presence and growth of pathogenic biological agents in an
individual host organism. In certain cases, infectious diseases may be
asymptomatic for much or even all of their course in a given host. In the
latter case, the disease may only be defined as a "disease" (which by
definition means an illness) in hosts who secondarily become ill after contact
with an asymptomatic carrier. An infection is not synonymous with an infectious
disease, as some infections do not cause illness in a host.
Infectious pathogens include some
viruses, bacteria, fungi, protozoa, multicellular parasites, and aberrant
proteins known as prions. These pathogens are the cause of disease epidemics,
in the sense that without the pathogen, no infectious epidemic occurs.
The term infectivity describes the
ability of an organism to enter, survive and multiply in the host, while the
infectiousness of a disease indicates the comparative ease with which the
disease is transmitted to other hosts. Transmission of pathogen can occur in
various ways including physical contact, contaminated food, body fluids,
objects, airborne inhalation, or through vector organisms.
Infectious diseases are sometimes
called "contagious" when they are easily transmitted by contact with
an ill person or their secretions (e.g., influenza). Thus, a contagious disease
is a subset of infectious disease that is especially infective or easily
transmitted. Other types of infectious/transmissible/communicable diseases with
more specialized routes of infection, such as vector transmission or sexual
transmission, are usually not regarded as "contagious," and often do
not require medical isolation (sometimes loosely called quarantine) of victims.
However, this specialized connotation of the word "contagious" and
"contagious disease" (easy transmissibility) is not always respected