When you get sick, you generally don't contract enough bacteria
at once for them to succeed in battling your immune system,
right? Their numbers must gradually increase in the host's body
before they know that they can attack. How does that work? I
think the current answer to this for bacterial infections is
quorum sensing. Quorum sensing is a signalling pathway in
bacteria which senses a molecule that the bacteria themselves
secrete. When the concentration of the quorum signal reaches a
certain level, the bacteria interpret this as their population
density reaching some threshhold. Bacteria are always around -
even infectious Staph, as described in the other answer, the
bacteria are always being cleared out by the immune system, but
when they find the right place where they can get critical mass,
they dig in, form a biofilm and secrete toxins, which can help
them divide more successfully. This is a description of the
process from a paper on Staph infection, a common bacterial
infection in humans. Viruses are