with the drugs that have been extensivley studied, medical controlled studies, none have a direct effect either way, on immunity. this includes; oxandrolone, anadrol, testosterone, winstrol and nandrolone.
theoretically all steroids could inadvertently boost immunity by way of lean mass gains, metabolic stimulation and improved mood.
"test flu" is more likely a result of contaminants or excess ba in UG products. it could also be your immune system thinking that the substance is an "invader" so it sets in to fight it off with a fever.
trenbolone doesn't lower immunity per se. it's ill effects can be attribited to its potency and ill fit with human biology. there's a reason that it's never been ok'd for human use in any country in the world.