It is unknown how this parasitic vegetation first arrived on this planet as it is not of native origin, but one can easily theorise that it arrived with the martian scouting expedition that crashed in the jungles of South America. The occupants of the scouting craft were at least traumatised enough to go into a deep hibernation if not killed outright. Whichever state they were in it is obvious that death for Martians is by no means as final a condition as it is for us native inhabitants of this planet. The expedition of explorers that discovered the mythical 3 sided pyramid were unfortunate enough to also locate the burial chamber of its occupants and become hosts for the Martian intelligences that lay dormant within.
From there it is easy enough to trace the vector of transmission of this parasitic plant as the Martian Hosts carried either spores or seeds back across the ocean, nurturing the plant-life once they had established their new identities. It is known that the red weed was present in an hermetically sealed environment within the dwellings of the Martian Hosts at the time of the first Great Paris-London Air-Race. From their the Red Weed vector is a more difficult to track precisely.
One vector is certainly a Professor Thomas Crapper, a participant in the Air Race who took samples of the germinated seeds from the Martian Hosts residence whilst attempting to obtain incriminating evidence of the Martian Conspiracy. Professor Crapper used the seeds to infect rats with the Red Weed Spores and study its behaviour as a parasite. Noting the virulence of the red weed and its complete dominance of the host nervous system, Professor Crapper returned to England with the Red Weed Samples and propagated them for further study within a sealed environment at his Warehouse.
The second vector by which Red Weed became present within London can be pieced together from recently discovered documents obtained from the French Criminal Mastermind, Fantomas. Discovering that he had been beaten to the theft of the Targetting Calculation Engines for the Intercontinental Verne Cannons (which he had planned to sell to Prussian bidders) by the Martian Hosts (although he was unaware of their true identities, or the reason for the pre-emptive theft), Fantomas's agents followed the hosts to their Paris residence and then finding that they had been beaten there too by others who were also investigating these mysterious men, they sent one of their number to discover the identity of the investigators (the afformentioned Thomas Crapper and his accomplice Jason Huntingdon). He was the lucky one amongst the agents of Fantomas, as of the two remaining who entered the residence of the Martian Hosts, both emerged infected with Red Weed spores. One had been infected by transmission through a wound from a barb on a red weed plant, the other by air born spores spread when he attempted to burn the red weed crop. It is supposed the news of the events of that night and what had transpired with his agents reached the ears of Fantomas rather quickly. One can guess that Fantomas, rather than seeking a cure for his agents, took the opportunity to study the conditions of infection with a human host, and then seeing a chance to exploit their ill fortune, purchased the residence of the Martian Hosts (who had subsequently died in an aviation "accident" over Sussex) from the executor of their estate (including all contents) and harvested the remaining Red Weed crop.
Fantomas attempted to destroy Crapper's supply of red weed, hoping to remove any threat that Crapper's study of the plant might pose to his next scheme. Using an underground reservoir beneath Kew Gardens to cultivate and then process red weed into flax for use in newly minted Bank Notes, thus infecting a large portion of the financial capital and sending panic throughout the economy, simulataneously destablising the gold reserve by flooding it with an excess of gold, Fantomas hoped to bring the British Empire into financial ruin and anarchy. At the same time Fantomas was using addicts and derelicts as experimental hosts for the red weed plant. Thankfully this nefarious scheme was thwarted, but much was revealed by Fantomas's diabolical plans.
The plant is resistant to most forms of environmental damage, being able to sustain life on inhospitable extraplanetary atmospheres. It thrives predominantly on water, growing at a rapid rate of expansion and draining and choking any supplies of water that it is introduced to. The tough fibrous nature of the weed is resistant to anhydrous substance, acids and alkalis of most strengths as well as vacuum and extreme pressure. The best known method for destroying the weed appears to be "Old Chomondleys' Reliable", a particularly toxic weedkiller, of an undisclosed but unstable chemical composition.
The weed can propogate itself either through seedpods (in heavy atmospheres leading to a clumped close distribution and slow spread of the weed) or through spores (carried over a further distribution area in lighter atmospheres). The seeds or spores can germinate in either soil, water or even flesh, wherever there is a volume of water to serve as nutrient. A special nutrient that encourages particular properties in the plant has been used to retard the germination of the seeds, preventing its maturation into a full red weed until the nutrient has run out.
The weed can be parasitic and spread through a host nervous system, creating a hybrid creature and augmenting its physical attributes whilst suppressing its mental faculties. Those infected by red weed can be spotted by a particular red hue to their eyes, a threading of red veins from the point of infection and a weak constitution. An inclination to drink any available liquid in the vicinity has been noted prior to full weed infection as well.
Once the weed has integrated with the host nervous system the transformation is complete. The host grows thorn like protrusion from the palms of their hands, used to drain blood or liquid from around them. A swaying motion as if in an invisible breeze can be detected and aggressive tendancies as well as muscle growth and heightened agility has been seen in certain cases of infection (although this is believed to be resulting from the nutrients used on the weed). Extreme dehydration results in wounds spilling a red-dust like substance (powdered blood) also containing seeds.
Recovery from Red Weed infection is unheard of. It is probable that it can be retarded whilst a cure is sought, if a cure exists.