Question: Hello! Would someone have better immunity against lesser pathogens after exposure to deadlier ones?

Andrew Yool answered on 21 Jul 2020:

Martin’s correct – it’s unlikely that the immunity gained from contact with one pathogen will confer to another one. This is because the rapid response our immune systems have to a return visit of a particular pathogen relies on it “recognising” this pathogen. This recognition occurs via the occurrence of antibodies in our blood after a previous infection. These are produced by a particular wing of the immune system known as B cells, and work by attaching themselves to specific molecules that pathogens carry (e.g. surface proteins), marking them up for destruction by other components of our immune system. The “specific” part of the last sentence is important – the shape of target molecules in pathogens allows fairly precise targeting. However, this means that if a new pathogen arrives, it’s not likely to present the same target, so will be missed by existing antibodies. But new antibodies will in time be made that identify pathogen molecules.
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The example that Martin gave – of smallpox and cowpox – is one of the founding stories of vaccination and is well worth reading up on. In this specific case, because the two viral diseases are related to one another (and present similar target molecules to the immune system), being exposed to one conveys immunity to the other. And since smallpox is far more dangerous, actively exposing people to cowpox has clear advantages.
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While that form of vaccination used a live (but less harmful) virus, modern vaccination can make use of non-infective components of viruses or even variants of a virus that are less dangerous (attenuated viruses). In all cases, however, the idea is to present your immune system with the kind of molecules that it would encounter when infected with the dangerous pathogen, and so raise an immune response ahead of you being infected.
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Vaccines work best where a pathogen always “looks the same” when it arrives, but if the molecules that identify it can change (i.e. and present a different shape), then a vaccine may be less effective. A good example here is seasonal flu – each year new vaccines are developed to cope with new strains (i.e. varieties) of this disease. However, I think we’re all hoping at the moment that COVID-19 is always boringly similar when it infects people – that way we’ve a much better chance of getting a good vaccine that will stop the pandemic in its tracks.