Microorganisms and Disease
The influenza vaccine is administered each year because

None of these
the vaccine is sufficiently toxic to make it necessary to administer only a small amount at any one time
mutations in the viral hemagglutinin may allow the virus to evade the immune response elicited by previous vaccines
it is a polysaccharide vaccine that does not confer long-term protection

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Microorganisms and Disease
A major difference between EHEC and EPEC is

EPEC passes through the placenta to infect the fetus and EHEC does not
EHEC possesses a type III secretion system and EPEC does not
EHEC secretes a Shiga-like toxin and EPEC does not
EPEC rearranges host cell actin and EHEC does not

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