Class 12 Biology Human Health Disease Protozoan diseases

Protozoan diseases


Caused by Plasmodium ( vivax, P. malariaeand P. falciparum).

Affects liver and RBC.

Transmits by biting of female anopheles mosquito which acts as a vector.

High fever occurring on alternate days, chill, vomiting are the common symptoms.

Malarial parasite requires two hosts to complete their life cycle-

  • Human
  • Anopheles mosquito.

Life cycle of malarial parasite.

  • Female Anopheles mosquito bites a healthy human and injects sporozoites (infective stage) with bites.
  • The parasites reach the liver by through blood and starts multiplying within the liver cells
  • Parasites then attack the red blood cells and reproduce asexually in the red blood cells and rupture the red blood cells which release a toxic substance called haemozoin responsible for chill and high fever recurring every 3 to 4 hours.
  • Some of the parasites differentiate into male and female gametocytes which are taken up by the mosquito during biting and sucking blood.
  • Formation of gametes and fertilization takes place in the intestine of mosquito.
  • The zygote develops and forms thousands of sporozoites which migrate into the salivary gland of the mosquito.
  • When the mosquito bites another human, the sporozoites are injected.


Fig. life cycle of malarial parasite  

Amoebiasis (amoebic dysentery)

Caused by Entamoebahistolytica.

Affects large intestine of man.

Transmitted by house flies which as mechanical carrier and food contaminated with cysts of Entamoeba

Constipation, abdominal pain, cramps, stools with mucous and blood clots are common symptoms.


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