Collagenous colitis (CC) is associated with non-bloody, watery diarrhea, which is pathophysiologically reasonable because normal colonic absorption (or excretion) of water and electrolytes can be blocked by the abnormally thick collagen layer in CC . However, CC has also been associated with six previous cases of protein-losing enteropathy (PLE), with no pathophysiologic explanation . The colon does not normally absorb (or excrete) amino acids/proteins, which is primarily the function of the small bowel . Collagenous duodenitis (CD) has not been associated with PLE . This work reports a novel case of CD (and CC) associated with PLE; a pathophysiologically reasonable mechanism for CD causing PLE (by the thick collagen layer of CD blocking normal intestinal amino acid absorption); and a novel association of PLE with severe COVID-19 infection (attributed to relative immunosuppression from hypoproteinemia, hypoalbuminemia, hypogammaglobulinemia, and malnutrition from PLE).