a few options;

also "asymptomatyczne (nosicielstwo / infekcja)"

see also Wikipedia, "subclinical shedding"



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Note added at 2 days1 hr (2009-06-21 21:37:33 GMT)
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(To Michal Bierski):
Citation format for this entry:

2 : an agent capable of transmitting a pathogen from one organism to another either mechanically as carrier (as houseflies that transport typhoid bacteria) or biologically by playing a specific role in the life cycle of the pathogen (as mosquitoes in relation to the malaria parasite) <fleas are vectors of plague> <aphids are vectors of plant viruses>

"vector." Webster's Third New International Dictionary, Unabridged. Merriam-Webster, 2002. http://unabridged.merriam-webster.com (21 Jun. 2009).