It is often quoted that a person needs to spend a significant amount of time in close proximity to a person with active TB to contract TB themselves. Even more specifically, it seems to be "common knowledge" (the worst sort of knowledge, in my opinion) that you would need to be on an plane for eight (8) hours with a TB sufferer to contract TB.
What seems to be overlooked is that that is a pure statistical calculation. Just as it doesn't mean that you will definitely contract TB after eight hours and one minute, it doesn't mean that you definitely won't contract TB after 7 hours and fifty-nine minutes.
It doesn't take a lot of logical thought to realize that, whilst the statistic is, perhaps, verifiable as having a statistical meaning, the conclusion being implied and drawn from the statistic is misleading. It actually implies some form of TB infection mechanism that is quite untrue. It implies some vague concept that the TB bacteria that you breath in is safe up to a certain level, after which you will contract TB (a type of "tipping point"). The truth is that the only way you can be sure that you will not contract TB is not breathe in a single TB bacterium. It is therefore quite possible that you could contract TB within the first minute of exposure. The longer the exposure lasts, the more likely you are to breath in a TB bacterium and contract TB.
Implementing a policy of only screening people who have spent eight hours in close proximity may make some sort of sense in the allocation of (all too small) TB medical resources but it does not guarantee the individual person that the system is protecting them from TB infection.
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