Infectious Gastroenteritis: This is the most likely diagnosis given the acute onset and high frequency of loose stools. A viral or bacterial infection is a common cause of gastroenteritis, which can lead to symptoms like diarrhea, abdominal cramps, and vomiting.

Irritable Bowel Syndrome (IBS): IBS is a chronic condition characterized by recurring abdominal pain and changes in bowel movements, which can include diarrhea. The long duration of symptoms (5 weeks) could suggest a chronic condition like IBS.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic diarrhea, abdominal pain, and weight loss. The frequency and duration of loose stools could indicate an inflammatory process.
Lactose Intolerance or Malabsorption: Difficulty digesting certain nutrients, such as lactose, can lead to chronic diarrhea. Other malabsorptive conditions, like celiac disease, can also cause similar symptoms.

Clostridioides difficile (C. diff) Infection: This bacterial infection can cause severe, life-threatening diarrhea and is often associated with antibiotic use. It's crucial to consider C. diff in patients with prolonged or severe diarrhea.
Giardiasis: An infection caused by the Giardia parasite, which can lead to chronic diarrhea and malabsorption if left untreated.
Colorectal Cancer: Although less likely in younger patients, colorectal cancer can cause changes in bowel habits, including diarrhea. It's essential to rule out this possibility, especially in older adults or those with a family history.

Whipple's Disease: A rare, systemic bacterial infection that can cause chronic diarrhea, weight loss, and malabsorption.
Short Bowel Syndrome: A condition resulting from a significant loss of intestinal tissue, leading to malabsorption and chronic diarrhea.
VIPoma: A rare pancreatic tumor that produces vasoactive intestinal peptide (VIP), causing severe, chronic diarrhea and dehydration.