END is difficult to keep out of the US because:

• A. Illegal bird smuggling can introduce the virus
• B. Some birds can be carriers
• C. Wild birds that cross into the US frequently carry this virus
• D. All of the above

Answer: (D)

END is hard to keep out of the US because there are multiple, overlapping routes for introduction. First, illegal bird smuggling can bypass border inspections and quarantine, allowing infected birds or contaminated materials to slip into the country and start an outbreak before any detection. Second, some birds can be carriers themselves, meaning they shed the virus or transmit it without showing obvious illness, so screening may miss these subclinical infections. Third, wild migratory birds cross borders frequently and can carry END; they mingle with local bird populations or contaminate shared water sources and environments, providing a continuous source of new infections. Taken together, these factors mean there isn’t a single fail-safe barrier, so all contribute to the difficulty of keeping END out of the United States.