There are over 400 different parasitic animals that can live inside human body.  From many different kinds of microscopic protozoa (amoeba) causing no symptoms, to large worms that can be several meters long, causing terrible joint pains.

Because there are so many different kinds of parasites, there are very many different signs and symptoms associated with having any of those animals inside of you. 
Just to illustrate: Tapeworm infection can cause Epileptic seizures, Ascaris infection can cause Asthma and even death, Malaria infection can cause fever and even death, sign of pin worms can be itchy nose, many different intestinal worms can cause or be a cofactor in Constipation, Chronic fatigue, Eczema, Psoriasis,  ... this list is really endless ...

Here are just some most common signs that doctors are looking for:

Possible signs and symptoms of internal parasites:

• Feel tired most of the time (Chronic Fatigue)
Feel tired most of the time (Chronic Fatigue)
• Have digestive problems? (gas, bloating, constipation or diarrhea that come and go but never really clear up)
• Have gastrointestinal symptoms and bulky stools with excess fat in feces
• Suffer with food sensitivities and environmental intolerance
• Developed allergic-like reactions and can’t understand why
• Have joint and muscle pains and inflammation often assumed to be arthritis
• Suffer with anemia or iron deficiency (pernicious anemia)
• Have hives, rashes, weeping eczema, cutaneous ulcers, swelling, sores, papular lesions, itchy dermatitis
• Suffer with restlessness and anxiety
• Experience multiple awakenings during the night particularly between 2 and 3 am
• Grind your teeth
• Have an excessive amount of bacterial or viral infections
• Depressed
• Difficulty gaining or losing weight no matter what you do
• Did a Candida program which either didn’t help at all or helped somewhat but you still can’t stay away from bread, alcohol, fruit, or fruit juices
• Just can’t figure out why you don’t feel really great and neither can your doctor
• itchy ears, nose, anus
• forgetfulness, slow reflexes, gas and bloating, unclear thinking;
• loss of appetite, yellowish face
• fast heartbeat, heart pain, pain in the navel;
• eating more than normal but still feeling hungry;
• pain in the back, thighs, shoulders;
• lethargy;
• numb hands;
• burning sensation in the stomach;
• drooling while sleeping;
• damp lips at night, dry lips during the day, grinding teeth while asleep;
• bed wetting;
• women: problems with the menstrual cycle;
• men: sexual dysfunction;

These are only possible symptoms, and please keep in mind that not everyone that has a few of these symptoms should automatically make the assumption that they are infected; however, if you suspect infection or if you have been unsuccessfully treated for any health problem, it might be smart to do some herbal parasite cleansing.