If a course of antibiotics or other chemotherapy is adminstered to a patient the delicate balance of the various flora is upset , so that complications can result. For example , diarrhea and intestinal upset might occur which could require consumption of yogurt food ( containing cultures of lactobacillus ) to reestablish and stabilize the microbial balance. Many antibiotics produced to combat bacterial infections are derived from fungi, but in recent years there has been an increasing frequency of fungal infections as a result of the widespread use of antibiotics. Antibiotics destroy the normal mucosal flora , allowing fungal 'weeds' ( e.g. the yeast causative of candidiasis ) to flourish without competition.

                   Algae may sometimes present hazards to human health. For example , if shellfish are ingested that have fed on red-pigmented dinoflagellates - then toxaemia may be caused with consequent muscle weaknesses or even death due to paralysis of the respiratory muscles. Hence environmental health or other statutory authorities will need to ban shell fishing during certain seasons if there is an outbreak of 'red-tide algae' in local coastal waters.                                                

                     Both on salt-water and inland fresh-water the blue-green algae (also called cyanobacteria) can cause poisoning because they excrete toxins which damage the liver or nervous system of animals and humans. It was reported from Brazil in April 1996 that 43 people died as a result of suspected algal poisoning of water used in kidney dialysis. The toxin then was microcystin-LR , which is known to be the most common toxin found in British algal blooms. During 1995 more than 200 toxic blooms of algae were reported from Britain's fresh-waters, and it is expected that the incidence will continue to rise in future years during extremes of hot summer weather. At such times swimming and fishing should be prohibited in infected waters.                                                                       

                    Protozoa phylum is subdivided into four subphyla _ each of which contains pathogenic members that are among man's parasites. Malaria , endemic in many tropical and subtropical lands, is the commonest arthropod-borne disease in the world. The malarial parasite is a protozoon spread by the bites of mosquitoes. Several thousand instances of the infection are reported each year in the U.K. amongst travellers from abroad. The genus trypanosoma includes three members known to be pathogenic to humans, and all are transmitted by biting insects. Amoebic dysentry infection occurs through taking water or food contaminated with cysts. Poor sanitation and food hygiene, assisted by flies, make such contamination possible. The amoebae attack the intestine (causing diarrhoea), and then the liver - resulting in hepatitis or liver abscesses. Pharmacological treatments that have been developed include the use of arsenicals, quinine derivatives , emetine , metronidazole or other drugs to clear the intestinal or hepatic amoebiasis.                                                                          

                     Helminths are classified in the phyla of flatworms and roundworms, and although most are macroscopic - yet their identification often requires microscopic examination of the eggs; and therefore microbiologists include them within their scope of study. Some of these parasites (e.g. cestodes / tapeworms) may live only in the intestinal tract of infected humans, while others may be disseminated to muscles, lungs, liver, blood etc.                                      

                      Most worms common in the U.K. cause only mild symptoms and generally do not pose any threat to general health, but anthelmintic drugs are usually necessary because the body's natural defences against infection are not effective against worm infestations. Certain types of worm infestation must always be treated since they can cause serious complications, and in cases of threadworm it may be advisable for anthelmintic treatment to be given to all family members if reinfection is to be prevented. It is thought from endemiological study that possibly one fifth of all children in the U.K. are affected by threadworms at any given time.