Infections associated with diseases

Infections associated with diseases are those infections that are associated with possible infectious etiologies that meet the requirements of Koch's postulates. Other methods of causation are described by the Bradford Hill criteria and evidence-based medicine.

Koch's postulates have been modified by some epidemiologists, based on the sequence-based detection of distinctive pathogenic nucleic acid sequences in tissue samples. When using this method, absolute statements regarding causation are not always possible. Higher amounts of distinctive pathogenic nucleic acid sequences should be in those exhibiting disease, compared to controls. In addition, the DNA load should become lower with the resolution of the disease. The distinctive pathogenic nucleic acid sequences load should also increase upon recurrence.

Other conditions are met to establish cause or association including studies in disease transmission. This means that there should be a high disease occurrence in those carrying a pathogen, evidence of a serological response to the pathogen, and the success of vaccination prevention. Direct visualization of the pathogen, the identification of different strains, immunological responses in the host, how the infection is spread and, the combination of these should all be taken into account to determine the probability that an infectious agent is the cause of the disease. A conclusive determination of a causal role of an infectious agent for in a particular disease using Koch's postulates is desired yet this might not be possible.

The leading cause of death worldwide is cardiovascular disease, but infectious diseases are the second leading cause of death worldwide and the leading cause of death in infants and children.

Other causes
Other causes or associations of disease are: a compromised immune system, environmental toxins, radiation exposure, diet and other lifestyle choices, stress, and genetics. Diseases may also be multifactorial, requiring multiple factors to induce disease. For example: in a murine model, Crohn's disease can be precipitated by a norovirus, but only when both a specific gene variant is present and a certain toxin has damaged the gut.

List of diseases associated with infectious bacteria and viruses
A list of the more common and well-known diseases associated with infectious pathogens is provided and is not intended to be a complete listing.

Epidemiology
Infectious pathogen-associated diseases include many of the most common and costly chronic illnesses. The treatment of chronic diseases accounts for 75% of all US healthcare costs (amounting to $1.7 trillion in 2009).

History
One of first examples of systematic study of disease causation was Avicenna, in the tenth century. The history of infection and disease were observed in the 1800s and related to the one of the tick-borne diseases, Rocky Mountain spotted fever. The cause of viral encephalitis was discovered in Russia based upon epidemiological clustering of cases. The virus causing this illness was isolated in 1937. The rash typical of Lyme borreliosis was identified the early 1900s. Historically, some chronic diseases were linked or associated with infectious pathogens. <!--For an infectious pathogenic microbe that has been noted to frequently accompany a disease, there are several logical possibilities that can explain this observed association:


 * The pathogen is an "innocent bystander" that plays no causal role in the etiology of the disease, but for some reason is more prevalent in patients with the disease (perhaps because the disease compromises the immune response, for example).
 * The pathogen predisposes to disease development (increases the risk of getting the disease), but the pathogen does not cause the disease (for example, genital herpes increases the risk of acquiring HIV).
 * The pathogen is a necessary, but not sufficient, cause of the disease: in other words, the pathogen can cause the disease, but does so only in combination with one or more other causal factors (such as host genetic factors, or toxin exposure).
 * The pathogen is a direct and singular cause of the disease, but this causality has yet to be proven.

Determining whether a pathogen plays a causal role in a chronic disease is often difficult for the following reasons:


 * The time between contracting an infectious pathogen and the appearance of the first disease symptoms can be lengthy, sometimes decades.
 * An infectious pathogen may not cause disease in every person.
 * An infection may be asymptomatic in its acute phase (when first contracted), and so go unnoticed; symptoms may only appear much later — in the form of a chronic disease.
 * Sometimes, only specific strains of a pathogen are linked to a disease; other strains of the same microbe may be harmless.
 * A pathogen may precipitate the disease only in combination with one or more other causal factors.
 * There may be more than one pathogen that can precipitate a given disease.
 * A given pathogen may not always cause the same disease — infection with it may lead to one of several different diseases.
 * There may be pathogens that are not readily detectable that play a causal role in a disease.
 * For obvious ethical reasons, you cannot inoculate infectious pathogens into humans to see if these do cause the disease or not.
 * A pathogenic microbe may cause disease by relatively easy to track direct means, such as by infecting and destroying cells; or may cause disease via more complex and convoluted indirect means, such as through the damage created by inflammatory cytokines or autoimmune processes that are induced by the microbial infection (for example, tuberculosis infection induces an inflammatory cytokine that then itself causes severe tissue damage).
 * A pathogenic microbe may not necessarily be present in the diseased tissues or organs (bacterial toxins for example can travel and damage tissues at sites distant from the infection site; and inflammatory or autoimmune processes precipitated by infectious pathogens can also act at tissue sites far removed from the infection).-->