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Typhus and typhoid fever are distinct infections, despite their overlapping fever, prostration and delirium. In the British historical comparison, typhus chiefly means epidemic louse-borne typhus, not every disease now called a typhus fever.

Classical typhus often begins relatively abruptly with severe headache, fever, prostration and a conspicuously ill mental state. Its eruption is commonly a more extensive dusky maculopapular rash on the anatomical trunk and limbs and may become petechial. Crowded conditions and infestation with body lice support the epidemiological distinction. Modern knowledge identifies Rickettsia prowazekii and transmission involving infected louse material. Injury to small-vessel endothelium helps explain the widespread illness; it is not chiefly a local ulcerating disease of the ileal lymphoid tissue.^[ref]

Typhoid fever more often develops insidiously, with persistent fever, headache and progressive weakness, together with abdominal discomfort, distension and either constipation or diarrhoea. Sparse, transient rose spots may occur on the anatomical trunk. Splenic enlargement and intestinal complications are important. The characteristic pathological process involves intestinal lymphoid tissue, especially Peyer's patches in the terminal ileum, with enlargement, necrosis and ulceration. Severe ulcers can bleed or perforate.^[ref]

Modern causation separates typhoid fever, due to Salmonella Typhi, from louse-borne typhus. Typhoid fever spreads through food or water contaminated by human excreta, including material from carriers; it is not acquired merely because an air space is crowded. A prolonged course, abdominal findings and intestinal haemorrhage favour it, whereas abrupt severe illness with a widespread dusky rash and louse exposure favour typhus.^[ref]

Neither the rash nor the bowel habit is invariable, and the old contrast between an approximately two-week typhus and a longer enteric illness is a tendency, not a diagnostic clock. An absent rose spot does not exclude typhoid fever, and an absent rash does not exclude typhus. Modern confirmation uses organism-appropriate investigations, including blood culture for enteric fever and appropriate serological or molecular testing for typhus. The important historical achievement is the separation of the syndromes; the later microbiological explanation should not be projected into every 1864 diagnostic account.

The classical contrast is between ulceration centred on intestinal lymphoid tissue in enteric fever and granulomatous, often circumferential ulceration in intestinal tuberculosis. The historical expression continued fever covered more than one illness. The characteristic bowel lesions meant here point especially to typhoid fever; they should not be attributed indiscriminately to typhus or every prolonged fever.

In typhoid fever, Peyer's patches and solitary lymphoid follicles, particularly in the terminal ileum, enlarge and become infiltrated. Their breakdown and necrosis lead to sloughing and ulceration. The larger ulcers characteristically follow the long axis of the intestine, reflecting the shape of the affected aggregated lymphoid patch on the side opposite the mesentery. Depending on their stage, one may see swollen patches, necrotic sloughs, open ulcers or healing surfaces. Depth varies; erosion into a blood vessel can cause haemorrhage, while full-thickness destruction can cause intestinal perforation and peritonitis.

Microscopically, the important cellular component includes swollen macrophages with ingested organisms and cellular debris; this is not simply a dense collection of neutrophils forming an ordinary abscess. The surrounding mucosal architecture may become severely disturbed. These current cellular descriptions refine the gross lesion recognized before the causative organism was identified.^[ref][ref]

In historical phthisis, bowel disease frequently meant intestinal tuberculosis associated with pulmonary tuberculosis. Lesions commonly involve the terminal ileum and ileocaecal region, although other sites occur. Nodules in mucosal or submucosal lymphoid tissue undergo caseous necrosis, coalesce and break through the mucosa. The ulcerated surface tends to spread transversely and may encircle the bowel. The edges can be irregular and undermined, with additional tuberculous granulomas in the wall, serosal surface or draining lymph nodes. Progressive fibrosis may narrow the lumen and produce a stricture; thickened masses as well as ulcers can occur.^[ref]

A section from a tuberculous ulcer may show central caseous necrosis bordered by epithelioid macrophages, giant cells and lymphocytes. A section from an enteric-fever ulcer instead emphasizes destruction of lymphoid patches with macrophage-rich infiltration. A practical anatomical examination records site, orientation, depth, edges, intervening mucosa, serosal nodules and draining lymph nodes, rather than judging one detached fragment in isolation.

Longitudinal lymphoid-patch ulcers favour the classical enteric-fever pattern; transverse, caseating and often stenosing lesions favour intestinal tuberculosis. These are useful patterns rather than absolute rules. Modern microbiological culture, organism-specific molecular tests and appropriately sampled histology can establish the cause when gross appearances overlap.