An infectious disease or communicable disease is caused by a biological agent such as by a virus, bacterium or parasite.

Infectious diseases are the invasion of a host organism by a foreign replicator, generally microorganisms, often called microbes, that are invisible to the naked eye.

Microbes that cause illness are also known as pathogens. The most common pathogens are various bacteria and viruses, though a number of other microorganisms, including some kinds of fungi and protozoa, also cause disease. An infectious disease is termed contagious if it is easily transmitted from one person to another.

An organism that a microbe infects is known as the host for that microbe. In the human host, a microorganism causes disease by either disrupting a vital body process or stimulating the immune system to mount a defensive reaction. An immune response against a pathogen, which can include a high fever, inflammation, and other damaging symptoms, can be more devastating than the direct damage caused by the microbe.

Infectious disease requires an agent and a mode of transmission (or vector).

A good example is malaria, which is caused by Plasmodial parasites, chiefly Plasmodium falciparum but does not affect humans unless the vector, the Anopheles mosquito, is around to introduce the parasite into the human bloodstream.

The vector does not have to be biological. Many infectious diseases are transmitted by droplets which enter the airway (e.g. common cold and tuberculosis).

What is an infection?

An infection is the detrimental colonization of a host organism by a foreign species.

In infection, the infecting organism seeks to utilize the host's resources in order to multiply (usually at the expense of the host).

The infecting organism, or pathogen, interferes with the normal functioning of the host and can lead to chronic wounds, gangrene, loss of an infected limb, and even death. The host's response to infection is inflammation. 

A symbiosis between parasite and host, whereby the relationship is beneficial for the former but detrimental to the latter, is characterized as parasitism.

Wound colonization refers to nonreplicating microorganisms within the wound. Replicating organisms exist in infected wounds and tissue is injured. All multicellular organisms are colonized to some degree by extrinsic organisms, and the vast majority of these exist in either a mutualistic or commensal relationship with the host.

An example of the former would be the anaerobic bacteria species which colonize the mammalian colon, and an example of the latter would be the various species of staphylococcus which exist on human skin. Neither of these colonizations would be considered infections.

The difference between an infection and a colonization is often only a matter of circumstance. Organisms which are normally non-pathogenic can become pathogenic under the right conditions, and even the most virulent organism requires certain circumstances to cause a compromising infection. Some colonizing bacteria, such as Corynebacteria sp. and viridans streptococci, prevent the adhesion and colonization of pathogenic bacteria and thus have a symbiotic relationship with the host, preventing infection and speeding wound healing.

The variables involved in the outcome of a host becoming inoculated by a pathogen and the ultimate outcome include:

• Route of entry of the pathogen and the access to host regions that it gains
• Intrinsic virulence of the particular organism
• Quantity or load of the initial inoculant
• Immune status of the host being colonized

As an example, the staphylococcus species present on skin remain harmless on the skin, but, when present in a normally sterile space, such as in the capsule of a joint, will multiply without resistance and create a huge burden on the host.

How are infectious diseases diagnosed?

Diagnosis is initially by medical history and physical examination, and imaging (such as X-rays), but the principal tool in infectious disease is the microbiological culture.

A microbiological culture, or microbial culture, is a method of growing a microbial organism to determine what it is, its abundance in the sample being tested, or both. It is often used a tool to determine the cause of infectious disease by letting the agent multiply (reproduce) in predetermined media in laboratory. In a culture, a growth medium is provided for a particular agent. After inoculation of a specimen of diseased fluid or tissue onto the medium, it is determined whether bacterial growth occurs.

Types of cultures include:

• Blood culture
• Sputum culture
• Tissue culture
• Culture of various fluids such as pleural fluid and peritoneal fluid
• Urine culture

Certain agents cannot be cultured, for example most viruses.