The main reason certain foreign diseases decimate native human and plant populations is that the native populations have no natural resistance to the diseases.
Dutch Elm Disease: This is a fungal disease of elm trees. Three different strains of the Ophiostoma fungus move into the trees via three different species of bark-boring beetles. The beetles usually target weaker trees or existing openings in healthier trees. The tree can't send nourishment to its branches from the roots, the branches die, and eventually the tree starves because the top is dead.
The disease is believed to have originally come from The Netherlands to North America in the early 1900's, where it wiped out millions of trees in the United States and Canada because the native elm population had no natural resistance. Through the years since the first infestation, people have learned how to stop the spread of the fungus, and have developed more disease-resistant strains of the American Elm.
Potato Blight: This is a disease that affects potato plants and is caused by the Phytophthora fungus. In Europe in the 1840's (particularly Ireland), it wiped out millions of acres of potatoes and caused major famine.
This disease is also caused when the native plants have no natural immunity to an organism. Poor methods of farming left the plants susceptible and they succumbed. Over the years since the first breakouts, farmers have learned to not leave last year's potatoes in the ground or in piles, where they rot and provide the perfect breeding ground for the fungal spores. Scientists have also developed fungicides that prevent the disease from attacking the potato plants, and blight-resistant potato varieties.
Small Pox: A very communicable disease throughout the world, particularly Europe and Asia. This disease is spread by the Variola virus and can be fatal in humans, particularly when they have no natural immunity to it.
In North America, during the 15th Century, smallpox wiped out 80-90% of several tribes of Native American Indians because they had no natural immunity to the disease. In fact, in South America and Australia, the original inhabitants of both continents were devastated in like manner. Again these people had no natural immunities to the disease.
Smallpox is now a controlled disease through the use of vaccinations, cleanliness, and improved diets.
In summary, all three diseases mentioned wiped out plants or humans because there was no natural immunity present to fight the diseases.