The number of single covalent bonds that most nonmetallic elements can form . is equal to the number of electrons needed to make a total of eight

can be any number between one and eight

is determined by subtracting eight from their total number of electrons

can be any even number between one and eight

A vector it is any animated object, capable of transmitting an infectious agent from its reservoir to another living being, in an active way, printing a certain sense to the transmission according to its characteristics.

Biological vectors are all those organisms in which some essential phase of the life cycle of the pathogen takes place and they are able to transmit it actively to man and animals, either by bite or through urine or feces.

Many of these vectors are hematophagous insects that ingest pathogenic microorganisms along with the blood of an infected carrier (person or animal), and then inoculate them to a new carrier by ingesting their blood. Mosquitoes are the best known vectors, although ticks, flies, sandflies, fleas, triatomines, rodents and some freshwater snails are also found.

Biological vectors are studied as a cause of disease, but also as possible cures for humans.