I think I have had influenza twice, based on my symptom pattern and the fact I was exposed to flu cases. But every winter I care for flu patients, so I have probably been exposed to flu viruses repeatedly each season. But I generally stay well. Why?
For each infectious disease, an "infectious dose" has been calculated. This refers to the number of microorganisms it takes, roughly, to cause infection in a healthy person. Ten bacteria are thought to be enough to cause a case of tularemia, while an estimated fifty thousand spores are needed to cause a case of inhalation anthrax.
So it is likely I got less than my infectious dose of flu viruses, repeatedly: small amounts that were quickly destroyed by my immune system. An infectious illness could be said to be an event characterized by exposure to a sufficient number of microorganisms to overwhelm immune protection.
Thinking about flu virus exposures that are insufficient to cause infection, one wonders whether they are enough to produce immunity. Maybe one or several of these exposures, over a period of time, serve as immune "boosters" to prevent subsequent disease when exposed to a larger amount. Is this the explanation for why I stay well?
If so, maybe instead of injecting attenuated (weakened) viruses, killed viruses or parts of viruses using standard vaccines, we could instead give people very tiny, repeated doses of live virulent viruses that could be inhaled. Live viruses produce a more long-lasting and stronger immunity than killed viruses, in general. (OTOH, inhaled, attenuated Flumist vaccine virus is less effective than killed vaccine in most adults.) By using the same route for this new form of vaccination as the route of infection, we would likely produce stronger immunity at the most important sites. Finally, we would avoid an injection, which bypasses our body's mucosal defenses, inserting materials like mercury, aluminum and formaldehyde deep into the body. Our mucosae exist to keep them out.
Maybe using appropriate infection control measures for H1N1 prevents some of us from getting the tiny exposures that lead to natural immunity, which might be more effective at preventing infection than a "killed" vaccine. Or maybe tiny, repeat exposures are common for health professionals, but don't often occur in the general population. Would repeated, controlled exposures to tiny doses be safe and efficacious? It would be useful to know.