In short: you will live a long healthy life.

If you get diagnosed and start effective medication, many people will live as long as a HIV-negative person.

As the treatment of HIV improves, so does the life expectancy of people living with HIV. Someone who is diagnosed at an early stage of infection, who has access to treatment and who follows the treatment guidelines will likely have a normal life-expectancy.

About one in three gay men find out that they are HIV-positive after the point at which it is currently recommended that treatment is started [1] (gay men are less likely to be diagnosed late than other key groups). One recent study found that life expectancy of people with HIV in the UK is decreased by 13 years [2] however this doesn't mean all people with HIV will live 13 years less than expected. The reduction in life expectancy is largely because many are diagnosed late which causes them to have a shorter life expectancy.  The earlier you are diagnosed, the better your chances of having a normal life-expectancy.

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1 Public Health England (PHE). HIV in the United Kingdom: 2014 report. November 2014.
2 May MT, Gompels M, Sabin CA. Impact on life expectancy of late diagnosis and treatment of HIV-1 infected individuals: UK CHIC. Journal of the International AIDS Society, 2010;13(Suppl 4):O27.