Most spiders are harmless and in fact are beneficial because they prey upon flies, crickets, and other insects. They generally will not attempt to bite humans unless held or accidentally trapped. Of the hundreds of species found in the United States, only the black widow and brown recluse are dangerous. Fortunately, both are relatively uncommon, and have markings that can be used to distinguish them from other non-threatening species.

These plants are the most common cause of allergic contact dermatitis in the United States. All contain the potent antigen urushiol, which will sensitize 60% to 80% of the persons who are exposed to it. This volatile oil may be carried on the fur of pets, clothing, shoes, toys, tools, or other objects and then transferred to the skin. Approximately 24 to 36 hours after a sensitized person is exposed to the urushiol, a blistery, itching rash develops. Usually within 15 minutes of contact, the urushiol binds to skin proteins. If it is washed off with soap and water before that time, a reaction may be prevented. After the antigen is fixed, however, it cannot be washed off or transferred to other areas. Scratching or oozing blisters cannot spread the antigen to other areas of the body or to other persons. New lesions that appear a few days after the primary lesions represent less sensitive areas where less antigen was deposited.