This sounds - at best - “counter-intuitive.” It seems that there would be a much LOWER risk of health issues if you REDUCE immigration of people from countries with exotic diseases and third-rate health-care systems.
Brexit will be irrelevant to immigration to the UK from the third world. The UK has always controlled its own borders - it isn't part of the Schengen open-borders area, which only includes some EU countries, not the UK. The UK refused to participate in Merkel's plan to allocate Syrian refugees across the EU on a quota system. Responsibility for third-world immigration to the UK has always lain with successive UK governments, not the EU.
The only form of immigration affected by Brexit is that by existing citizens of other EU countries moving to Britain under the 'freedom of movement' rules, not immigration from outside the EU..