In some parts of Africa they use basil plants to protect themselves from mosquitos, quite successfully.

Mosquitos home in on humans by detecting the smell of lactic acid in our muscles. But the scent of basil confuses them because it masks the smell of humans. Basil is one of the few plants that emits it's smell without having it's leaves touched. They plant it around doors and windows to ensure the mosquitoes don't fly in.

In India Basil is considered a sacred herb and is planted at the doors of temples. It is probably because some wise guru realised it repelled insects centuries ago and incorporated it into cultural and religious practice to ensure everyone had basil around.

Of course a nice side effect of basil is that it is a great herb for cooking, especially Italian cooking!