What we colloquially call "seed ticks" are actually the two juvenile forms ("larvae" and "nymphs") of larger ticks. They are extremely small, but a bite from one can cause big problems! They can still carry diseases like Lyme and Ehrlichia, and due to their tiny size they are easily overlooked by even the most diligent pet owners, particularly in long-coated animals. For this reason, we recommend all dogs and cats be on a quality tick-preventative year-round, and that dogs be screened for common tick-borne diseases annually (unfortunately there aren't any affordable in-house screening tests for tick-borne diseases in cats yet). Your local vet can advise you on which preventative products work best in your area.
I caught this one crawling one me after walking a dog out on the clinic lawn. And we hadn't even gone near the woods or tall grass!