This warning is very true. I have noticed a greater amount of poision oak in the greater drainage area during my hikes. For those who don't recognize it, here is a discription of what to watch out for...

*Western poison oak (Toxicodendron diversilobum, Anacardiaceae) This deciduous perennial has highly variable growth habits: vine, ground cover, shrub. In chaparral it usually grows as an erect shrub. Its compound leaves have three leaflets. The terminal leaflet ranges in size from 1 to 13 cm. (1/3 to 5 inches) long by 1 to 8 cm.(1/3 to 3 inches) wide. Lateral leaflets may be half as long. Leaves appear shiny bright green or red in spring then turn dull green or dusty red in summer. The leaves are very resinous. They can be confused with blackberry leaves which also have three leaflets. However, western poison oak leaves and branches lack any spiny or hairy projections, and the leaflets tend to have more rounded tips. The tiny yellowish green flowers occur in racemose panicles from April through May. These are followed by 11/2- to 6-mm. (up to 1/4-inch) creamy white to brownish berry-like fruits (drupes) which are an important food for wildlife. Western poison oak is indifferent to serpentinitic soils. It occurs in canyons and on slopes in chaparral and woodland communities. TOXIC: This plant causes severe contact dermatitis.
Toxicodendron means "poison tree." Diversilobum refers to the various shapes the leaves take.

As you will note in the discription, these may be in bloom right now and being so may draw the local bugs to their flowers, so the bugs in the area can also carry the oils on their legs. There is an old addage to remember: Leaves of three, let them be. You can easily google for pictures of what they look like since many varieties have slightly different leaves.