Many parasites, including a good proportion of helminths, but also some protozoan parasites common in developed countries, such as Giardia intestinalis and Cryptosporidium, are causing some kind of intestinal illnesses. As such, it is perfectly normal that these parasites caused many different effects on your stomach. Actually, they are susceptible to cause a wide range of gastrointestinal symptoms, such as random abdominal pain, diarrhoea, constipation and vomiting. As such, while infected with some parasites, it is quite frequent to experience intermittent periods of diarrhoea and constipation. Parasites can also cause the infected person to feel bloated or to experience more gas. However, as these symptoms are quite unspecific, they can be easily misdiagnosed as something else. It is noteworthy that in some extreme cases, too many helminths in your guts are susceptible to cause a bowel obstruction usually requiring urgent surgery to remove the excessive amount of worms, prevent bursting and restore the normal functions of the intestine.
Other gastrointestinal symptoms are also susceptible to occur when infected with a parasite. For example, the presence of blood in the faeces could be one of them. However, this symptom is not specific to parasitic diseases and can have a lot of other causes. On the other hand, some gastrointestinal symptoms are usually more specific of parasitic infections. For example, it is the case of sudden weight loss combined with an increase in appetite explained by the fact that the worms are taking their share of the food ingested by the host.
Furthermore, some parasitic infections like the one involving Giardia intestinalis have similar symptoms to the irritable bowel syndrome. This affection is characterised by a lot of different stomach and intestinal symptoms. As the real causes of this syndrome are still unknown, active infections with some protozoan parasites have been suggested to be one of the possible causes.
Finally, a lot of different diseases can also have an effect on your stomach. Furthermore, these effects can be simply caused by a more mechanical issue such as a digestive problem. As such, even though stomach effects are often involved in parasitic diseases, especially involving intestinal parasites, having these symptoms are not specific to them. However, if these stomach symptoms last since a long time and that none other possible causes have been found in your tests, it could be interesting to investigate the possibility of parasitic diseases.