"The unusually high temperatures experienced in the UK this summer has seen a number of outbreaks of Koi Herpes Virus (KHV) in hobbyist Koi ponds, as well as in natural bodies of water. There have been reported outbreaks in several trade premises, believed to originate in fish imported from Israel, and in the past 12 months the USA has seen several outbreaks among Koi imported from Japan."
I don't believe a hobbyists quarantine is always for the best. Ask the dealer where the fish are sourced from, and what measures are taken to prevent the spread of KHV and only deal with people you trust.
Not all dealers quarantine koi in a way that would trigger latent viruses.