Rakweed

Rakweed is an infectious plant from the planet Raxacoricofallapatorius.

Biology
The Rakweed was a delicacy on Raxacoricofallapatorius, which the natives frequently ate.

Initially the plant is an inert, shrub-like growth, but when exposed to sunlight, it blooms, shooting thousands of spores into the air. The spores served two purposes: to increase the Rakweed's number, and also to infect life forms which were considered 'dangerous' to the plant. Those who were affected by the spores would suffer a distinctive red and black rash, and enter into a deep coma which they may never return. The spores also "communicated" with each by generating a high-pitched screeching sound. This connection could be broken by another high-frequency sound, such as the noise made by a school bell, which would result in the destruction of all plants and spores within the sound's range.(SJA: The Gift)

History
The Slitheen-Blathereen family gave a potted Rakweed to Sarah Jane Smith as a gift for stopping the Slitheen, which she reluctantly accepted. She immediately went to scan it using Mr Smith, but apparently he could find no harm in it. Unknown to Sarah Jane, the Rakweed flowered the next morning, and as a result it spread - much to the Slitheen-Blathereen's content. The spores' first victim was Sarah Jane Smith's son Luke Smith.

The Rakweed spread quickly around the local area, threatening to engulf the whole of London in a matter of hours. However, Rani and Clyde discovered the plant's weakness, prompting Sarah Jane to have Mr. Smith transmit the sound of the bell across all audio systems in the affected area.

The Slitheen-Blathereen were addicted to Rakweed and would gorge themselves upon the food. Sarah Jane later used this to her advantage, having Mr. Smith replicate the bell's noise, destroying the Rakweed in the Slitheen-Blathereens' stomachs. This resulted in the creation of a deadly amount of methane gas, causing the Slitheen-Blathereen to explode (or, as Clyde put it, "fart themselves to death"). (SJA: The Gift)