On December 9, 2020, a 27-year-old male patient, who is a student, presented with chief complaints of "loose stools for 4 months, intermittent low-grade fever for 1 month", and he was admitted to the Departments of Gastroenterology, The First Affiliated Hospital of Wannan Medical College. His complaints did not include abdominal pain, abdominal distension, bloody purulent stool, cough, expectoration, chest tightness and shortness of breath. And a physical examination revealed no abnormalities. This patient has just returned from Japan (November 2, 2020) and has been quarantined for 14 days twice. In Japan, colonoscopy (October 7, 2020) showed multiple ulcers between the sigmoid colon to the terminal ileum (Figure 1A-D), and pathology found that inflammatory cell infiltration in intestinal mucosa and epithelioid granuloma in submucosa (Figure 1E). Regular use of mesalazine did not significantly improve the symptoms of loose stools. Meanwhile, taking levofloxacin orally was not effective for his fever. 
Laboratory investigations revealed a normocytic anaemia (hemoglobin 92g/L), hypoproteinemia (albumin 28.8g/L), raised inflammatory markers (C-reactive protein (CRP) 72.9mg/L, erythrocyte sedimentation rate (ESR) 74mm/h) and thrombocytosis (PLT 675*10^9/L). Examination of pathogenic microorganisms suggested that rubella virus immunoglobulin G (IgG) was 208.5IU/mL and cytomegalovirus IgG was 184.4U/mL, which were exceeds the normal limit. Fecal bacteria proportion showed that gram-positive coccus 10%, gram-positive bacillus 10%, gram-negative bacillus 80%, and no fungal spores were found. Fecal occult blood (OB) is negative, and no obvious abnormality was observed in tuberculosis antibody (TB-AB), tuberculous infection with T cells (T-sport), purified protein-derived tuberculin (PPD), blood coagulation, routine before blood transfusion and tumor markers.
The next day (December 10, 2020), the patient developed a high fever with a body temperature of 39.8 C. A chest high resolution computed tomography (HRCT) was urgently performed and result showed infectious lesions in the upper lobe of the right lung and the lower lobe of both lungs, with multiple patchy ground-glass shadows and nodular ground-glass shadows, which suggested interstitial pneumonia (Figure 1F). Then, multiple novel coronavirus (2019-nCoV) pneumonia nucleic acid test was performed, and the results were all negative. Despite the empirical antibiotic and antiviral therapy that was started, his fever remained.
On the third day after admission (December 12, 2020), laboratory investigations revealed that fecal OB was positive and hemoglobin dropped from 92g/L to 84g/L. On the other hand, fecal general flora, Salmonella and Shigella culture were all negative. Acid-fast bacilli and general bacterial cultures in sputum were also negative. Gastroscopy showed chronic superficial gastritis. A total gastrointestinal CT angiography showed that segmental leaping thickening in part of jejunum, distal ileum, ileocecal, ascending colon and transverse colon, proliferation of peripheral small vessels, multiple enlarged lymph nodes around the diseased intestine and at the root of mesenteric vessels (Figure 1G). Pelvic magnetic resonance revealed no abnormalities, which suggested that the patient was without an internal fistula.
Based on the above evidences, this patient was finally diagnosed with CD, according to the World Gastroenterology Organization. According to the Montreal classification of IBD and Harvey and Bradshaw's simplified Crohn's disease activity index (CDAI) method, this patient was classified as A2L3B1, and his CDAI was 433.
Then, we used steroids 50mg/d, and the temperature was returned to normal quickly. A follow-up chest CT after 5 days (December 15, 2020) showed that multiple patellar ground glass shadows were almost completely absorbed under the pleura of the lower lobe of both lungs (Figure 1H). Then, infliximab was used. We can see the inflammatory markers were decreased gradually; however, nutritional status indicators were increased gradually. Meanwhile, fecal OB was negative and stool property returned to normal. A repeat colonoscopy after 4 months (April 19, 2021) revealed multiple hyperplastic small polyps in the terminal ileum, ascending colon, transverse colon and descending colon, with some scars (Figure 1I-L). And repeat laboratory investigations after 4 months showed that hemoglobin was 132g/L, platelet was 253*10^9/L, albumin 44.3g/L, CRP 0.88mg/L, ESR 4.7mm/h. He gained 10kg in weight.