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People can be infected by bacteria, viruses, fungi and animal parasites, but are any human or animal diseases caused by plants? Is it possible to suffer a moss infection, come down with a bad case of the ferns, or contract wisteria? If not, why have no plants taken advantage of us in this way?
鈥� Plants do cause disease. Think of allergies to plants such as poison ivy or products such as peanuts. (also known as the tree of heaven) bears flowers whose smell has been associated with headaches and nausea.
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鈥淧lants do cause disease. Think of allergies to poison ivy or to plant products such as peanuts鈥�
But these are not infectious diseases. Plants do not become pathogens inside us as they need light for photosynthesis, which they can鈥檛 get inside our bodies.
Eric Kvaalen, La Courneuve, France
鈥� I know of no vascular plants that are truly infectious in animals, though some do infect other plants. H. G. Wells wrote a short story entitled The flowering of the strange orchid in 1894, but that was really about plants preying on humans.
Hairs on some fern species may be carcinogenic if eaten regularly, while pollens may cause hay fever. Cereal crops may host disease-causing fungi such as ergot and aspergillis, and various plants host and transmit pathogenic invertebrates, such as flukes.
Certain barbed grass seeds can catch in animals鈥� hair and sometimes actually burrow through skin. I have seen one that had germinated in the kidney of a little girl. However, the seeds of most plants are too large to be effective as infectious agents; spores might do better. It鈥檚 also conceivable that evolution might eventually lead to parasitic characteristics in dust-like seeds such as those of witchweed or orchids, or in pollen.
Still, I don鈥檛 think that these examples capture the spirit of the question.
Jon Richfield, Somerset West, South Africa
鈥� There is a case, widely blogged about, of a man with a small fir tree growing in his lungs (). He was cutting pine trees when he inhaled the seed of a cone, which settled in his lung. Soon his body started to form a cyst around the sapling. Doctors feared it was cancerous so opted to remove it, whereupon they found the sapling.
Christopher Payne, Essex, UK
鈥� My late uncle, an English country doctor, used to tell the story of a patient with excruciating pain in his face. The culprit turned out to be a tomato seed lodged in a crevice in the roof of his mouth, which had germinated and was growing into his hard palate.
Andrew Cooper, Walls, Shetland, UK