In the field of medicine, particularly epidemiology, "vector" refers to an organism which acts as an intermediate agent in transmitting disease.
For example:
"In Central and South America certain species of the Mexican Bedbug are the *vectors* for a highly fatal disease known as Chagas disease.". The preceding quote was taken from the node assassin bug by gnarl. (The emphasis on "vectors" is mine)
The first time I encountered this use of the word "vector" was during an annual visit from the ADEQ. I had begun a project to try and compost sewage sludge from the small wastewater treatment plant which was my responsibility to oversee and operate. This compost would be used in the landscaping of the park, which I also supervised. The two ADEQ guys asked what the big ugly pile was next to the sewer plant. I explained its purpose and they looked at each other and one said, "Vectors?" The other one nodded.
After a brief and friendly discussion during which they explained about flies and larvae and potential disease transmission, we arrived at a simple solution. They said a tarp over the pile would solve the problem. Relieved, I told them to consider it done. We passed the inspection with flying colors and, (pauses for effect), I learned a new meaning for the word "vector".