Human Infectious Diseases

Pathogen-disease associations

A pathogen-disease association links a causative organism with the clinical condition it produces, and exam questions usually test whether you can recall this link precisely. Many diseases are named after a clinical feature rather than the organism, so the name alone can mislead. Scarlet fever is caused by Streptococcus pyogenes, which produces an erythrogenic toxin that gives the characteristic skin rash. Undulant fever, a recurrent febrile illness with a wavy course, is caused by Brucella species, which are intracellular Gram-negative coccobacilli often acquired from animals or unpasteurised dairy products. Pott's disease is tuberculosis of the spine, caused by Mycobacterium tuberculosis. Traveler's diarrhea is most often caused by enterotoxigenic Escherichia coli, which produces heat-labile and heat-stable enterotoxins. Matching all four pairs correctly requires memorising each organism with its clinical picture, not just one pair in isolation.

Atypical pneumonia is a clinical category of community-acquired pneumonia caused by organisms that lack a typical cell wall or grow inside host cells. Because these organisms are not reached by beta-lactam antibiotics and do not grow on routine culture media, the disease is called atypical. Important causes include Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella pneumophila and Coxiella burnetii. Typical bacterial pneumonia, by contrast, is caused by organisms such as Streptococcus pneumoniae, Klebsiella pneumoniae and Haemophilus influenzae, which present with lobar consolidation or bronchopneumonia and respond to beta-lactams.

Diseases are classified by the type of causative agent. Protozoal diseases are caused by unicellular eukaryotic parasites such as Plasmodium, Entamoeba and Leishmania. Kala-azar, or visceral leishmaniasis, is a protozoal disease caused by Leishmania donovani and transmitted by the bite of infected female sandflies. Cholera (Vibrio cholerae), gonorrhea (Neisseria gonorrhoeae) and syphilis (Treponema pallidum) are bacterial diseases, so recognising the agent type is the key to eliminating wrong options.

Diagnostic assays for infection

Diagnostic assays detect either the pathogen, its products, or the host's immune response to it. Each assay has a specific target, and exam questions often pair an assay with its application. The Limulus amoebocyte lysate test uses blood cells extracted from horseshoe crabs, which clot (gel) in the presence of endotoxin, the lipopolysaccharide of Gram-negative bacterial outer membranes. It is therefore used to detect endotoxin contamination, for example in injectable drugs.

The Weil-Felix reaction is a serological test for rickettsial infection. It relies on cross-reacting antigens of certain Proteus strains (OX19, OX2 and OXK) that share epitopes with rickettsiae, so patient antibodies agglutinate these Proteus antigens. The test is therefore a classical aid in diagnosing rickettsial diseases such as typhus, although it is less specific than modern assays.

Serological tests such as the complement-fixation test detect antibodies in patient serum by measuring whether complement is consumed in an antigen-antibody reaction. In the pairing given in the key, this test is applied to detecting IgM. The Oakley-Fulthorpe test is a serological assay used to determine antigen-antibody specificity. Distinguishing these assays by their target (endotoxin, antibody class, specificity, or rickettsial cross-reaction) is the core skill for matching questions.