Pennsylvania State Forests are inhabited by multiple species of rattlesnake.  Rattlesnake bites are rarely fatal.  IF we see one, and IF we don't hear the rattle, and IF we get too close, and IF it strikes, and IF it's not a dry bite, what to do
Pa. DCNR Dept info on Timber Rattlesnakes. 

PDF Brochure: "Rattlesnakes in Pennsylvania State Forests" <-- right-click,  choose "save target as.."

Copperheads

Cottonmouth aka Water Moccasin ---NOT FOUND in N.E. Pennsylvania normally

it is found throughout Florida, with a range extending north to Virginia and west to Illinois, Missouri, Oklahoma, and Texas. Its habitat encompasses wetlands and waterways, such as streams, springs, rivers, lakes, ponds, marshes, swamps, sloughs, reservoirs, retention pools, canals, and even roadside ditches. Occasionally it can be found rather far from the water, and has been found in trees and bushes. Cottonmouths feed primarily on fish, frogs, mice, rats, and other small mammals.

Often, cottonmouths are confused with water snakes, non-poisonous snakes who share the same habitats. These harmless water snakes are often killed out of fear and ignorance. Actually, cottonmouths are also often killed for the same reasons. Except for self defense or positive identification in the case of snakebite, it is best to leave all snakes alone. Snakes help keep the environment in balance. For instance, a shortage of snakes means an increase in the mice and rat population, which brings on epidemics of disease often fatal for humans. Also, picking up or attempting to kill a snake can be one sure way of being bitten.

Found in the eastern United States from extreme southeastern Virginia, south through the Florida peninsula and west to Arkansas, southeastern Kansas, eastern and southern Oklahoma, and eastern and central Texas. A few records exist of the species being found along the Rio Grande in Texas, but these are thought to represent disjunct populations, now possibly extirpated. The type locality given is "Carolina," although Schmidt (1953) proposed that this be restricted to the area around Charleston, South Carolina.[1]

Campbell and Lamar (2004) mentions this species as being found in Alabama, Arkansas, Florida, Georgia, Illinois, Indiana, eastern Kentucky, Louisiana, Mississippi, Missouri, North Carolina, Oklahoma, South Carolina, Texas, and Virginia.[4] Maps provided by Campbell and Lamar (2004) and Wright and Wright (1957) also indicate its presence in eastern Tennessee and extreme southeastern Nebraska.[4][2]

In Georgia it is found in the southern half of the state up to a few kilometers north of the fall line with few exceptions. Its range also includes the Ohio River Valley as far north as southern Illinois, and it inhabits many barrier islands off the coasts of the states where it is found.

Black Bears
** The last time we were at World's End State Park, we saw a cub.  It was walking across the road as we approached World's End.  Mama bear was close by.

Many Pennsylvania state parks are habitat for black bears. Although they appear cute and cuddly like a teddy bear, black bears are wild animals.

A black bear can scramble up a tree like a raccoon and sprint as fast as a race horse. Bears use their claws to tear apart rotting logs to find food, and those claws also work well to open garbage cans and coolers. The size and strength of a black bear is astonishing.

Black bears have poor eyesight and fair hearing, but an excellent sense of smell. Aromatic scents coming from your food can attract a curious and hungry bear from a great distance.

Store all food items inside a vehicle. At primitive, walk-in campsites, suspend food between two trees, ten feet in the air and three feet from either tree.

Black bears normally avoid people, but bears dependent on eating human food can become slightly aggressive when people get between them and food.

If you come in contact with a black bear, try chasing it away by making loud noises like yelling, honking a car horn or banging a pot. Notify a park employee if you have difficulties with bears.

Never approach a bear and be especially wary of mother bears and cubs.

Ursus americanus 
     The black bear ranges across forested Canada from Newfoundland to British Columbia as well as much of the United States. A solitary animal most of the year, they pair up briefly during the mating season. Cubs remain with their mother for about a year, who protects which prevents them from being killed by the adult males. 
     Black bears swim well and often climb trees to feed on buds and fruit. They have a keen sense of smell, acute hearing, but poor eyesight. They can be seen at any hour of the day, but are most active at night. When very young, the cubs cry when afraid and hum when contented. 
     Black bears are omnivorous; their diet consists of about 75 percent vegetable matter, 15 percent carrion, and 10 percent insects and small mammals. Their love for honey is well known, and sweet, ripe corn in autumn is also a favorite.