They're not actually new, they're just new to some of their surroundings. They're bacteria and parasites that are expanding their reach as the planet warms. This is already happening. The World Health Organization estimated that in 2000 about 154,000 deaths around the world could be attributed to disease outbreaks and other conditions sparked by climate change. 154,000 and that's six years ago.
The Los Angeles Times reports that the earth has already warmed enough to bring trouble to northern regions from Sweden to Alaska. Bacterial blooms in Alaskan inlets have forced producers to change the way they raise and harvest shellfish. Sweden, meanwhile, is trying to cope with the northern migration of encephalitis-bearing ticks. What researchers are finding is that even incremental increases in temperature are expanding the distribution of bacteria, bugs and weeds.
The US Department of Agriculture has been testing how increasing CO2 levels affect weeds. They're testing ragweed with CO2 levels predicted for 2050. One researcher is quoted as saying, "It's like feeding a hungry teenager."