How mosquitos spread diseases
The blood protein is required to complete the mosquito's egg production sequence. Throughout the feeding procedure, the female penetrates her victims skin by her proboscis, (a long straw similar to structure with a pointed end) instills her saliva (which contains anti-coagulants) and then sucks the victim's blood in through her proboscis.
If the victim's blood has disease-causing organisms, they get also sucked into the mosquito’s stomach. These organisms are then preserved within the mosquito and sooner or later may be instilled into the next victim's bloodstream. This way the mosquito can spread virus from animal to animal, animal to human being, or else even from individual to individual.