Sickle-cell anaemia still persists in the population despite being harmful. This mutation can also be beneficial in certain conditions.The mutant Hb^S type haemoglobin is found at high frequencies (upto 20% and above) in the tropical Africa.
It is known that heterozygotes (Hb^S / Hb^A), having both types of haemoglobin show resistance to malarial infection as the body targets the Plasmodium falciparum infected cell for destruction. In contrast, individuals homozygous for normal haemoglobin
(Hb^A / Hb^A) suffer high mortality rates in early childhood due to malarial infection. Thus, the allele for sickle-cell has been maintained because heterozygotes have a higher reproductive success than either of the two possible homozygotes.

Differences between cancerous and normal cells are: 

The part of life cycle of Plasmodium spent in human body includes following stages:
(i) Sporozoites enter the human body through the bite of infected female Anopheles mosquito (vector)
(ii) From the blood, they enter liver cells and multiply within the liver cells.
(iii) Infected liver cells burst and release the parasites again in the blood.
(iv) They attack RBCs repeatedly, multiply and cause their rupture.
(v) Ruptured RBCs release a toxin called haemozoin which is responsible for high recurring fever and resultant chill/shivering.
(vi) When a female Anopheles mosquito bites an infected person, these parasites enter the mosquito body and undergo further development

(i) Differences between innate immunity and acquired immunity are:

(ii) Differences between active immunity and passive immunity are:

During changing weather, one is advised to avoid crowded places because changing seasons are the time when infectious agents are more active as moist conditions favours the growth of pathogens. Also, people are more vulnerable as their body system is busy in adapting the changing environmental conditions of temperature, humidity, etc.
Thus, getting easily infected by these pathogens.

Many microbial pathogens enter the gut of human along with food. The preventive barriers to protect the body from such pathogens are as follows:
(i) The mucus coating of the epithelium lining of the gut helps in trapping microbes entering the body.
(ii) Saliva in the mouth and hydrochloric acid in gastric juice secreted by stomach prevent microbial growth.

(a) Neutrophils and macrophages dispose off microbes and dead cell by feeding on them. Hence, they are called the soldiers and scavengers of the animal body.
(b) Consumption of alcohol brings blurred vision, slows down the response of body, impairs judgement and reduces coordination of limb muscles. Hence, one should avoid driving after intake of alcohol.
(c) Persistent use of corticosteroids is harmful because it may cause osteoporosis.

(a) Fertilization takes place in the mosquito intestine.
(b) Female Anopheles in the stomach.
(c) Female Anopheles mosquito release sporozoites in human body.
(d) Human body, in liver and blood cells