Gutworms and similar parasites are present in untreated drinking water in developing countries, and were present in the water of developed countries until the routine chlorination and purification of drinking water supplies.  Recent research has shown that some common parasites , such as intestinal worms (., hookworms ), secrete chemicals into the gut wall (and, hence, the bloodstream) that suppress the immune system and prevent the body from attacking the parasite.  This gives rise to a new slant on the hygiene hypothesis theory—that co-evolution of humans and parasites has led to an immune system that functions correctly only in the presence of the parasites. Without them, the immune system becomes unbalanced and oversensitive.  In particular, research suggests that allergies may coincide with the delayed establishment of gut flora in infants .  However, the research to support this theory is conflicting, with some studies performed in China and Ethiopia showing an increase in allergy in people infected with intestinal worms.  Clinical trials have been initiated to test the effectiveness of certain worms in treating some allergies.  It may be that the term 'parasite' could turn out to be inappropriate, and in fact a hitherto unsuspected symbiosis is at work.  For more information on this topic, see Helminthic therapy .
Allergic reaction: The hypersensitive response of
the immune system of an allergic individual to a substance. When
an allergen enters the body, it causes the body's immune system to
develop an allergic reaction in a person with an allergy to it. This
reaction can occur when the immune system attacks a normally harmless
substance (the allergen). The immune system calls upon a protective
antibody called immunoglobulin E or IgE to fight these invading
substances. Even though everyone has some IgE, an allergic person
has an unusually large army of these IgE defenders -in fact, too many
for their own good. This army of IgE antibodies attacks and engages
the invading army of allergic substances of allergens. As is often
the case in war, innocent bystanders are affected by this battle.
These innocent bystanders are special cells called mast cells. When a
mast cell is injured or irritated, it releases a variety of strong
chemicals, including histamine, into the tissues and blood that
promote allergic reactions. These chemicals are very irritating and
cause itching, swelling, and fluid leaking from nearby cells. These
allergic chemicals can cause muscle spasm and can lead to lung airway
and throat tightening as is found in asthma and loss of voice. They
are also what leads to the familiar hay fever or allergic rhinitis and
common pink eye .