vector
A foreign gene on its own usually cannot get into a cell, survive there, or get copied — it needs a ride. A vector is that vehicle: a DNA molecule that carries a foreign gene into a host cell and allows it to be maintained and replicated there.
Good vectors share a few features. They can enter or be introduced into cells, they replicate once inside (or insert themselves into the host genome so they are copied along with it), and they include a way to detect and select cells that took up the vector, such as a marker gene. Many also carry convenient restriction sites for inserting the gene and, in expression vectors, a promoter to switch the gene on.
Different jobs call for different vectors. Plasmids are the everyday choice for small inserts in bacteria; bacteriophages, cosmids, and artificial chromosomes carry larger pieces; and modified viruses are often used to deliver genes into animal or plant cells, including in some gene therapies. The vector is chosen to match the host and the size of the cargo.
To study a human gene in mouse cells, a researcher inserts it into a viral vector, which then ferries the gene across the cell membrane and into the genome.
Match the vehicle to the destination: viruses excel at entering animal cells.
In genetics, vector means a DNA carrier of genes; in epidemiology the same word means an organism, like a mosquito, that carries a pathogen. They are unrelated meanings that happen to share a name.