Koch's postulates have been modified by some epidemiologists, based on the sequence-based detection of distinctive pathogenic nucleic acid sequences in tissue samples.
Higher amounts of distinctive pathogenic nucleic acid sequences should be in those exhibiting disease, compared to controls.
Other conditions are met to establish cause or association including studies in disease transmission.
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.
[2] Other causes or associations of disease are: a compromised immune system, environmental toxins, radiation exposure, diet and other lifestyle choices, stress, and genetics.
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.
[15][16] Colorectal cancer is associated with the bacteria Helicobacter pylori, Streptococcus bovis and Fusobacterium nucleatum,[17] with human papillomaviruses,[18] and with the helminth Schistosoma japonicum.
[26] Lung cancer is associated with the bacterium Chlamydia pneumoniae,[27] with human papillomaviruses, and with Merkel cell polyomavirus.
Major depressive disorder is associated with bornavirus,[50] as well as Bartonella[51] and Borrelia[44] species bacteria.
The cause of viral encephalitis was discovered in Russia based upon epidemiological clustering of cases.