Necrotic spot virus and tomato spotted wilt virus are a big problem currently with Impatiens in particular - usually present in the plants when you buy them, only showing no symptoms. Caused by Western Thrips, which came here in 1980s, carrying both these viruses - the thrips gets killed over winter, except in glasshouses, so any stock from glass houses (which is all impatiens) may well be infected. All this info from RHS.
Otherwise, they can suffer from verticillium wilts if the area they're planted in is where you planted them year on year.

Great reply from bamboo. Mine too have suffered and I will not buy any from Garden centres next year, but I wonder if Impatiens grown from seed could be affected in the same way? Will watch out for any further comments.

In theory not, Breda, but they would need to be grown in a heated glass house nonetheless, even if its yours. There is some rumour going round about a new virus, but I've been unable to verify that, so perhaps they're talking about the two I've already mentioned.