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If a person contracts a virus, viral conjunctivitis for example, is it possible for the individual to become "reinfected" with the exact same strain of the virus once the person has it treated and the symptoms have gone away? I am a nursing student and am fascinated with virology, but my basic microbiology class did not go very in-depth on the subject.

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The nature of infectious agents is that they transmit between organisms. This means that they have reservoirs outside a single host, and hence of course reinfection with the exact strain is possible if this strain leaves the body and at a later stage is carried back to it. This could typically happen for example with the common influenza, spreading from one child to other children at school and later being carried back to the original child through them.

This is what the adaptive immune response counters: Immune cells specific to a pathogen will proliferate and differentiate both into effector cells to destroy the pathogen, and into memory cells. These can mount a much stronger immune response much faster when they come in contact with the same (or a structurally very similar) pathogen, meaning that a secondary infection normally results in weaker or no symptoms, or at least a quicker regress of the infection.

Reinfection immunity is complicated by evasion mechanisms pathogens have evolved. In viruses the most prominent one is probably high mutation rates of those antigens which tend to be targeted by immune systems. This means that reinfection with the same strain of virus can still trigger an immune response like a completely unknown pathogen. Again, influenza is a great example. The reason why we get flu again winter after winter and why the influenza vaccine is not commonly distributed*, is because this virus tends to mutate over the year with relatively low infection prevalence. Then during the winter, when human immune systems are generally weakened already by the temperature, typically only one or two strains cause epidemics - because we were maybe immune to last year's influenza, but this year's has mutated antigens which our immune system does not recognise yet.

*) Influenza vaccine exists. It is developed new every year and is distributed among the population at risk: in the UK for example, people older than 65[if I recall that poster correctly] are encouraged to have the vaccination annually. It is not given to younger people as the vaccination itself causes symptoms, i.e. "flu". Younger people would easily resist the actual influenza and hence it is a better bet simply to hope they do not get infected at all. Older people however face a realistic risk of death from the virus and hence the vaccination can save lives.

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