Seal the lid but allow air holes still. This fungus is dangerous but will not be contracted via air. I am looking into some natural fungicides on your behalf methuselah and checking there ingredients against dangerous chemicals towards ants.
Feed them and still give them water, carry on as normal I will contact some people I know and see if they have encountered the problem, the ants MAY remove the fungus themselves, they moved out of the nest to prevent further infection and spread.
DO NOT LET THE INFECTED NEAR OTHER ANT SPECIES, this could cause a massive environmental disaster as L. Niger and other British species are not immune to this infection and most have never encountered it.
The fungus will not harm humans, but please don't try
I will reply in under 24 hours.
EDIT: Ants also spread with their grooming movements a range of potent antibiotics, that are secreted from glands in their bodies, which may act as fungicides.