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I mean how are the sporozoites able to swim in the reverse direction of blood flow through the proboscis of the mosquito into the human body?

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Nice question! Basically, the mosquito does not only suck; it also injects some saliva into the host with its saliva glands. From National Geographic:

The mosquito starts salivating as soon as it probes the [host]’s skin, releasing substances that prevent blood vessels from constricting, stop blood from clotting, and prevent inflammation. Sometimes, Choumet could see the saliva as small bubbles that hung around the tips of the mouthparts. And even after the mosquito stops feeding, pockets of saliva linger in the lower layers of the skin.

And from cdc.gov:

After 10-18 days, the parasites are found (as "sporozoites") in the mosquito's salivary glands. When the Anopheles mosquito takes a blood meal on another human, the sporozoites are injected with the mosquito's saliva and start another human infection when they parasitize the liver cells.

The probiscus is actually equipped to inject saliva as well as to suck blood. And once the saliva has entered the victim, it can spread to the whole body since the blood is constantly moving. From mosquitnoband.com:

Mosquitoes also have special hematophagous arthropod saliva in their proboscis. Hematophagous arthropod saliva is a scientific way of saying “spit from a blood-sucking creature with an exoskeleton.” This specific type of saliva has chemicals and proteins in it that prevents blood from clotting. So what do mosquitoes do with their hematophagous arthropod saliva? They inject it into their victim before they start sucking its blood to prevent any clotting while they slurp.

Hope this helps!

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