That may be because the parasitic organisms adapt better than their hosts, which are more complex organisms, say scientists.
Thomas Raffel with the University of Oakland said he and colleagues with the University of South Florida subjected 80 Cuban tree frogs to varying temperatures in a lab setting. The amphibians were then also exposed to the fungus Batrachochytrium dendrobatidis (Bd), which is infectious and can be fatal for the animals.
The scientists found that after exposure to the Bd, the infection rate was higher for a group of frogs kept in a 25 Celsius environment for four weeks and then transferred to 15 C incubators, compared to a group of frogs that never shifted from the 15 C climate.
In another experiment, when the frogs were moved between 25 C and 15 C climates according to a natural day-night cycle, they were mostly able to resist the parasitic fungus.
Parasites acclimate more easily
But once the variations in temperature became unpredictable, the scientists said the frogs weren't able to acclimate as easily, making them vulnerable to infection from the fungus.
"Increases in climate variability are likely to make it easier for parasites to infect their hosts," Raffel told Reuters.
"If you shift the temperature, a frog is more susceptible to infection than a frog that is already adapted to that temperature."
The research, published in the latest edition of the journal Nature, theorizes that the Bd parasites — as smaller organisms — are able to adapt more easily to climatic shifts owing to their size and because they have fewer cells.
"This might allow parasites to exploit periods of suboptimal host immunity following unpredictable temperature shifts," the report says.
Factoring in size, life expectancy and the metabolisms of the frogs, the researchers estimated that it takes 10 times longer for the host frogs to adapt to the unpredictable temperature fluctuations than it does for the parasites.
Raffel added that further study is needed to confirm how climate change might also affect human parasites such as tape-worm or malaria, as this research examined one species of tropical frog.
Warm-blooded creatures could also be less susceptible to parasites than cold-blooded creatures after unpredictable temperature fluctuations.