Aye, there's the rub. There are critters that will pass through a 2 micron pore that you absolutely do NOT want in your body. Crypto, etc. is way smaller and .3 micron, while possibly needing greater than gravity pressure to use is closer to "safe". With most filters, pre-filtering to get the logs and boulders out of the untreated water really increases the time between filter cleaning or replacing. "Pre-filtering" can be a bandanna, coffee filter, or even just letting the logs and boulders settle out in a pot for 15-20 minutes before filtering.
The fastest my filter (MSR) ever got clogged was when one of my Scouts did an "experiment" to find out how efficient it was. He deliberately stirred up the pool of water we were using. Sure enough, within 1-2 quarts of water (waaaaaay less than we needed that hot and humid afternoon) it clogged and needed cleaning. I made him take it down and do the cleaning.
IIRC crypto cysts are about 5 microns, so a 2 micron filter should suffice for them. Giardia cysts are even larger.