Some plants may be dangerous to your cats; find the list here.
L-Lysine is a supplement that can be given to cats to help controlling herpes virus (a form of cat flu). Many cats contracted the virus when kittens and although they may not be sick, they are still carriers of the virus, which can break out if their immune system is low and in times of stress. Giving L-Lysine to your cats will help them to fight against the virus. It is especially useful in the situation of feral cat colonies, where the cats are not vaccinated and more susceptible to become sick. It can be bought in paste form from vets, but also in tablet form from health stores, which may be a more economical solution when you have a large number of cats. Read more about L-Lysine here or here.
Community Cats Network 