Re: How is it possible to become infected with the same organism more than once

Very interesting question.
(I hope the snow has stopped in the Chicago area.)

   The Organisms Side
   First, some pathogens continually change or mutate their protective
capsules or coat.   This is the case for influenza, where there are two
major serotypes A and B. The A tends to be more severe and we usually
include two A substrains in each year`s vaccine and only one B strain. Of
course, the virus changes its coat proteins e.g., the  the hemagglutinin
each year and in so doing can fool the immune system. In this case you may
have nice immunity to last year's strain, including antibodies and 
cytotoxic T cells but the mutated strain can change its coat to avoid being
neutralized (eliminated). What we need is a broad based vaccine that is
given by the nasal route and which will induce a long-lasting mucosal and
systemic immune response that will protect you year in and year out. By the
way, just such a vaccine is in final phase 3 trials and will be available
for the public soon.

   Why are booster shots necessary for vaccines like cholera or tetanus ?

   Let`s use tetanus as the example since we really do not have a suitable
cholera vaccine.

   Infants receive their DPT shots, which includes tetanus toxoid at 2, 4
and six months of age. The reason for this multiple injection is two-fold.
First, the babies will have various levels of  antibodies to tetanus toxin
from their mothers. The titer will vary and if the titer is high the first
immunization of the baby will not work. Instead of immunizing, the baby's
anti- tetanus toxin antibodies will simply bind to the vaccine and
eliminate it.  Thus, one keeps immunizing until the baby  starts to make
his/her own immune response. The second point is also interesting. Tetanus
toxoid vaccine is excellent; however your immune response will wane without
boosting. You still have some memory T and B cells but you cannot respond
fast enough to make anti-toxin antibodies to protect you if you encounter
the organism Clostridium tetani that produces this powerful exotoxin. The
trick is that you need to have a certain level of circulating antibodies to
the exotoxins to protect you. If you do not, the organism can produce
enough toxin to kill you before your memory T and B cells can expand and
make a new antibody response.