From Longman Dictionary of Contemporary Englishbe down with somebodybe down with somebodyspoken informal to be friends with someone → down
Examples from the Corpus
be down with somebody• All the senior police officers were down with flu - so the Chief Constable asked for his aid.• By the time I jumped back inside he looked like he was down with flu.• These kids are 13,14, and they wan na be down with somethin'.• When they arrived, they were told that Mahdi was down with the flu. 