Bacterial overgrowth can cause a variety of symptoms, many of which are also found in other conditions, making the diagnosis challenging at times. Many of the symptoms are due to malabsorption of nutrients due to the effects of bacteria which either metabolize nutrients or cause inflammation of the small bowel, impairing absorption. The symptoms of bacterial overgrowth include nausea, flatus, constipation, bloating, abdominal distension, abdominal pain or discomfort, diarrhea, fatigue, and weakness. SIBO also causes an increased permeability of the small intestine. Some patients may lose weight. Children with bacterial overgrowth may develop malnutrition and have difficulty attaining proper growth. Steatorrhea, a sticky type of diarrhea where fats are not properly absorbed and spill into the stool, may also occur.

Patients with bacterial overgrowth that is longstanding can develop complications of their illness as a result of malabsorption of nutrients. Anemia may occur from a variety of mechanisms, as many of the nutrients involved in production of red blood cells are absorbed in the affected small bowel. Iron is absorbed in the more proximal parts of the small bowel, the duodenum and jejunum, and patients with malabsorption of iron can develop a microcytic anemia, with small red blood cells. Vitamin B is absorbed in the last part of the small bowel, the ileum, and patients who malabsorb vitamin B can develop a megaloblastic anemia with large red blood cells.

In older adults, small bowel bacterial overgrowth is associated with a higher frequency of diarrhea, a lower body mass index, and a significantly lower serum albumin concentration.

Small intestine bacterial overgrowth (SIBO), also termed bacterial overgrowths, or small bowel bacterial overgrowth syndrome (SBBOS), is a disorder of excessive bacterial growth in the small intestine. Unlike the colon (or large bowel), which is rich with bacteria, the small bowel usually has fewer than 10,000 organisms per millilitre. Patients with bacterial overgrowth typically develop symptoms including nausea, bloating, vomiting, diarrhea, malnutrition, weight loss and malabsorption, which is caused by a number of mechanisms.

The diagnosis of bacterial overgrowth is made by a number of techniques, with the gold standard being an aspirate from the jejunum that grows in excess of 10 bacteria per millilitre. Risk factors for the development of bacterial overgrowth include dysmotility; anatomical disturbances in the bowel, including fistulae, diverticula and blind loops created after surgery, and resection of the ileo-cecal valve; gastroenteritis-induced alterations to the small intestine; and the use of certain medications, including proton pump inhibitors.

Small bowel bacterial overgrowth syndrome is treated with an elemental diet or antibiotics, which may be given in a cyclic fashion to prevent tolerance to the antibiotics, sometimes followed by prokinetic drugs to prevent recurrence if dysmotility is a suspected cause.

Most of the symptoms of BLS are non specific but nevertheless warrant the utmost attention. These include:

- Loss of appetite

- Nausea

- Flatulence

- Diarrhea

- Fullness after a meal

- Fatty stools (steatorrhea)

- Unintentional weight loss

- Generalised weakness

As a result of the concomitant vitamin and mineral deficiencies that occur as a result of the malabsorption associated with BLS patients with advanced cases should be investigated for:

- Vitamin B12 deficiency

- Folate deficiency

- Iron deficiency

- Vitamin E deficiency

Blind loop syndrome (BLS), commonly referred to in the literature as small intestinal bacterial overgrowth (SIBO) or bacterial overgrowth syndrome (BOS), is a state that occurs when the normal bacterial flora of the small intestine proliferates to numbers that cause significant derangement to the normal physiological processes of digestion and absorption. In some cases of blind loop syndrome, overgrowth of pathogenic non-commensal bacteria has also been noted. It has long been understood that from birth, and throughout life, large amounts of bacteria reside symbiotically within animal gastrointestinal tracts such as the human gastrointestinal tract. The understanding of this gut flora has even led to novel treatments for bowel irregularity that utilize so called "probiotics" or good bacteria that aid in normal digestion.

The problem of BLS arises when the bacterial colonies residing in the upper gastrointestinal tract begin to grow out of control or are altered in their makeup thereby creating a burden on the normal physiological processes occurring in the small intestine. This results in problems inclusive of but not restricted to vitamin B12 deficiency, fat malabsorption and steatorrhea, fat-soluble vitamin deficiencies and intestinal wall injury.

The primary symptoms of IBS are abdominal pain or discomfort in association with frequent diarrhea or constipation and a change in bowel habits. Symptoms usually are experienced as acute attacks that subside within one day, but recurrent attacks are likely. There may also be urgency for bowel movements, a feeling of incomplete evacuation (tenesmus), bloating, or abdominal distension. In some cases, the symptoms are relieved by bowel movements. People with IBS, more commonly than others, have gastroesophageal reflux, symptoms relating to the genitourinary system, chronic fatigue syndrome, fibromyalgia, headache, backache, and psychiatric symptoms such as depression and anxiety. About a third of men and women who have IBS also report sexual dysfunction typically in the form of a reduction in libido.

IBS can be classified as either diarrhea-predominant (IBS-D), constipation-predominant (IBS-C), or with alternating stool pattern (IBS-A) or pain-predominant. In some individuals, IBS may have an acute onset and develop after an infectious illness characterized by two or more of: fever, vomiting, diarrhea, or positive stool culture. This postinfective syndrome has consequently been termed "postinfectious IBS" (IBS-PI).

The classic symptoms of coeliac disease include pale, loose, and greasy stool (steatorrhoea) and weight loss or failure to gain weight. More common symptoms are subtle or primarily occur in organs other than the bowel itself. It is also possible to have coeliac disease without any classic symptoms whatsoever. This represents at least 43% of the cases in children. Many adults with subtle disease only have fatigue or anaemia.

The diarrhoea that is characteristic of coeliac disease is (chronic) pale, of large volume, and abnormally bad smelling. Abdominal pain and cramping, bloatedness with abdominal distension (thought to be due to fermentative production of bowel gas), and mouth ulcers may be present. As the bowel becomes more damaged, a degree of lactose intolerance may develop. Frequently, the symptoms are ascribed to irritable bowel syndrome (IBS), only later to be recognised as coeliac disease; a small proportion of people with symptoms of IBS have underlying coeliac disease, and screening for coeliac disease is recommended for those with IBS symptoms.

Coeliac disease leads to an increased risk of both adenocarcinoma and lymphoma of the small bowel (enteropathy-associated T-cell lymphoma (EATL) or other non-Hodgkin's lymphomas). This risk is also higher in first-degree relatives such as siblings, parents, and children. Whether or not a gluten-free diet brings this risk back to baseline is not clear. Long-standing and untreated disease may lead to other complications, such as ulcerative jejunitis (ulcer formation of the small bowel) and stricturing (narrowing as a result of scarring with obstruction of the bowel).

Four rosacea subtypes exist, and a patient may have more than one subtype:

1. Erythematotelangiectatic rosacea exhibits permanent redness (erythema) with a tendency to flush and blush easily. It is also common to have small, widened blood vessels visible near the surface of the skin (telangiectasias) and possibly intense burning, stinging, or itching. People with this type often have sensitive skin. Skin can also become very dry and flaky. In addition to the face, signs can also appear on the ears, neck, chest, upper back, and scalp.

2. Papulopustular rosacea presents with some permanent redness with red bumps (papules); some pus-filled pustules can last 1–4 days or longer. This subtype is often confused with acne.

3. Phymatous rosacea is most commonly associated with rhinophyma, an enlargement of the nose. Signs include thickening skin, irregular surface nodularities, and enlargement. Phymatous rosacea can also affect the chin (gnathophyma), forehead (metophyma), cheeks, eyelids (blepharophyma), and ears (otophyma). Telangiectasias may be present.

4. In ocular rosacea, affected eyes and eyelids may appear red due to telangiectasias and inflammation, and may feel dry, irritated, or gritty. Other symptoms include foreign body sensations, itching, burning, stinging, and sensitivity to light. Eyes can become more susceptible to infection. About half of the people with subtypes 1–3 also have eye symptoms. Blurry vision and vision loss can occur if the cornea is affected.

Variants of rosacea include the following:

- Pyoderma faciale, also known as rosacea fulminans, is a conglobate, nodular disease that arises abruptly on the face.

- Rosacea conglobata is a severe rosacea that can mimic acne conglobata, with hemorrhagic nodular abscesses and indurated plaques.

- Phymatous rosacea is a cutaneous condition characterized by overgrowth of sebaceous glands. "Phyma" is Greek for swelling, mass, or bulb, and these can occur on the face and ears.