Malarial parasites are introduced into the blood of humans as sporozoites.

Sporozoites:
- Sporozoites are the infective stage of the malaria parasite that is transmitted to humans through the bite of an infected female Anopheles mosquito.
- When a mosquito carrying the malaria parasite bites a human, it injects the sporozoites into the bloodstream along with its saliva.
- The sporozoites travel to the liver, where they invade hepatocytes (liver cells) and undergo asexual replication.

Invasion of hepatocytes:
- Once inside the hepatocytes, the sporozoites differentiate into hepatic merozoites.
- The sporozoites undergo a series of transformations within the liver cells, eventually forming thousands of merozoites.

Merozoites:
- Merozoites are released from the infected hepatocytes and enter the bloodstream.
- They invade red blood cells (RBCs) and begin a new cycle of asexual reproduction.
- Within the RBCs, the merozoites multiply by a process called schizogony, where they undergo multiple rounds of nuclear division and cytoplasmic segmentation.
- This results in the formation of numerous daughter merozoites within the RBC.

Clinical symptoms and further development:
- The release of merozoites from the infected RBCs leads to the destruction of the RBCs, causing the characteristic symptoms of malaria, such as fever, chills, and anemia.
- Some merozoites differentiate into sexual forms called gametocytes, which can be taken up by mosquitoes during a blood meal.
- Once inside the mosquito's gut, the gametocytes undergo sexual reproduction, forming new sporozoites that migrate to the salivary glands of the mosquito.
- When the infected mosquito bites another human, it injects these sporozoites into the bloodstream, continuing the cycle of malaria transmission.

Conclusion:
In summary, malarial parasites are introduced into the blood of humans as sporozoites. These sporozoites undergo a series of transformations within the liver cells, forming hepatic merozoites. The merozoites are released into the bloodstream and invade red blood cells, causing clinical symptoms. Some merozoites differentiate into gametocytes, which can be taken up by mosquitoes, thereby completing the transmission cycle.