People are always worried about bears, rattlesnakes, poison oak and the like.
In 40 plus years of hiking around the high sierra the only problems I've heard of with plants or animals are mosquitoes and getting scratched up while bushwhacking thru fields of manzanita after getting lost.
Although a bear did grab a Tahoe neighbors dog by the head (The bear ran away after he threw a rock at it. and the dog was ok.) and a bear did break thru the screen door on our cabin when some renters left food on the table with only the screen door shut, I've never heard of a hiker attacked by a bear.

In 2013, after leading a trip where four participants got blisters, despite being warned about breaking in their boots, ..., I raised blisters to #1.

In "Hiking & Backpacking - A Trailside Guide", Karen Berger says,
"I'm not afraid of snakes or bears, I've never met a criminal in the woods. "For a long time I was very, very afraid of blisters."
"Blisters are probably the number-one cause of backcountry misery"

My current list of dangers for hiking/snowshoeing in the High Sierra is:

1. Blisters - (and bunions).
   
2. Sunburn See also Sunglasses
3. Dehydration/Heat exhaustion. See Heat Index
4. Rocks (sprained ankles, tripping, stubbing toes while wading without water shoes)
5. Mosquitoes :-)
6. Virusis picked up from other people due to lack of sanitation.
   In 2012 a man died in Yosemite from hantavirus
   Virusis are rarely present in the streams and lakes. Bacteria (Giardia, E. coli, ...) are not as big a problem. See "Bad Water" at the bottom of the list.
7. Getting lost
8. Dangerous cliffs, rivers, snow, and waterfalls
9. Bee or wasp stings
10. Hypothermia - In early October, 2008 a woman died from hypothermia on the Pacific Crest Trail above Lake Tahoe.
    See Wind Chill
11. Altitude Sickness
12. Lightning
13. Avalanche - In backcountry areas like Castle Peak - YouTube
14. Ticks
15. Poison oak
16. Forest Fires
17. Bears
18. Bad Water

Plants and Animals

What	Highest elevation	Notes
Poison Oak	5,000
Bears	10,000	Black Bears only; They are afraid of people. There are no grizzlies in California.
Mountain Lions	10,000	Sightings rare. They will avoid people.
Rattlesnakes	6-7,000	Rare at higher elevations. They are slow, rarely aggressive and rarely rattle.
Deer Ticks (Lime disease)	7,000	Tick season is brief and early - usually March-May. Usually at lower elevations.
Yellowjackets	10,000	These ground-nesting wasps are active mid-June through the fall

Safety Management:
Accidents fall into two categories, preventable and unpredictable. Rick Curtis, Director of Princeton U. Outdoor Action, has developed the Risk and Safety Management System (RASM), expanding on Alan Hale's work in the 1980s.

Links:
Emergency Contacts in the Tahoe Area
Outdoor problems under health
Forest Service Alerts and Notices
Trip Report: Buckwheat's Pacific Crest Trail (PCT) Thru-Hike Summary
Ratlesnakes - "You'll be walking down the trail and almost step on one, then it hisses and rattles at you while it slithers off the trail. Occasionally, a rattler will coil up and refuse to move off the trail, no matter what you try to do to it. Bites are actually extremely rare and often contain no venom at all. Still, encounters can be unnerving."

Safety Management Plans
Safety & Skills | Mountaineering Council of Scotland

Sierra Nevada National Forests Management Indicator Species Project

The Backpacker's Field Manual, Revised and Updated: A Comprehensive Guide to Mastering Backcountry Skills by Rick Curtis