Halo blight is a bacterial disease and is indeed seed borne. Both runner and french beans can be infected by it. Apart from home-saved seed, the commonest reason for it to manifest itself is when the seed is soaked in water for a few hours before planting (a very popular practice with peas and beans among some gardeners). One duff seed can then infect all the rest. However, if just one seed in the packet has the disease then, particularly in wet conditions the disease can spread rapidly from that one plant to all the other plants in the row.
The other basis of infection is the spread of the bacteria from diseased tissue debris on the soil surface from a previous crop, but this can obviously only be the reason if you had it last year as well.
Although the disease is not soil borne, because of the risk of re-infection from old plant tissue you should not plant beans on the same spot for another two seasons.
