A vector is something that helps transmit a disease to another organism. It does not cause the disease itself, but transmits bacteria or viruses to another organism. Typical vectors include mosquitoes and ticks. They feed on the blood of one organism, pick up the bacteria or viruses, and when they feed on a different organism, the pathogen is transmitted as well. Examples of vector transmitted diseases are Lyme (tick), malaria (mosquito), plague (fleas),and encephalitis (mosquito). Controlling diseases transmitted by vectors means reducing the numbers of vectors. This is why it is important to get rid of standing water--mosquitoes breed there. In the middle ages, cats were thought to be associates of witches and were killed. What happened instead is that with fewer cats, more rats were about--and the fleas carrying plague lived on the rats. In other words, getting rid of the cats made things worse!