Is plant fungus the cause of such things such as poison ivy and other infections?
Poison ivy is actually its own plant. The rash and itching caused by touching it is due to the oil that is on the leaves. About 75% of humans are allergic to the oil, it is a natural defense mechanism that the plant has developed. Since people and animals are allergic to it, they learn to avoid it, which means it is able to survive. An interesting side note is the Mythbusters show done on poison ivy and potential cures for the rash. They found that of all of their staff only two were allergic to poison ivy. This is an extremely unsual result given the percentages above. I have to wonder if perhaps the plant is losing its allergenic properties, or perhaps humans are becoming less sensitive to it over time.
