The pesticide, Permethrin, is often used to treat clothes so that they act as the first line of defense against insects. Some flea and tick medications for dogs contain permethrin. While permethrin is effective (and normally safe) for dogs, it can be very bad for cats. According to Wikipedia:

“Pesticide-grade permethrin is toxic to cats. Many cats die after being given flea treatments intended for dogs, or by contact with dogs having recently been treated with permethrin. In cats it may induce hyperexcitability, tremors, seizures, and death.”

When used properly permethrin is an effective defense against insects but be aware of the side-effects and risks of this chemical. It is often sprayed on clothes to make them insect resistant. Permethrin must be re-applied after clothes have been washed multiple times (will last for up to six washings).