Infectious colitis: This is the most likely diagnosis due to the acute onset of bloody diarrhea and fever, which are common symptoms of bacterial or viral infections affecting the colon.

Inflammatory bowel disease (IBD): Conditions like ulcerative colitis or Crohn's disease can present with bloody diarrhea and fever, especially if the disease is newly diagnosed or in an exacerbation phase.
Food poisoning: Certain types of food poisoning, such as that caused by Salmonella, Shigella, or E. coli, can lead to bloody diarrhea and fever.
Diverticulitis: Inflammation of a diverticulum in the colon can cause bloody diarrhea, fever, and abdominal pain.

Clostridioides difficile (C. diff) infection: Although less common in previously healthy individuals, C. diff can cause severe diarrhea and is a critical diagnosis to consider, especially if the patient has recently been on antibiotics.
Intussusception: A condition where a part of the intestine slides into an adjacent part, which can cause bowel obstruction and ischemia, presenting with bloody diarrhea and fever.
Ischemic colitis: Reduced blood flow to the colon can cause bloody diarrhea, fever, and abdominal pain, and is a medical emergency.

Neutropenic colitis (typhlitis): Typically seen in immunocompromised patients, this condition involves inflammation of the cecum and can present with bloody diarrhea and fever.
Radiation colitis: If the patient has a history of radiation therapy to the abdominal area, radiation colitis could be a consideration, although it's less likely in a 30-year-old without such a history.
Vasculitis affecting the gut: Certain types of vasculitis, like Henoch-Schönlein purpura, can cause gastrointestinal symptoms including bloody diarrhea and fever, but are relatively rare.