What everybody else said. Handwave it unless it is plot relevant.
For food and water, this would include:
- Deserts
- Artic adventures
- Lost at sea (drinkin' water becomes something of an issue if your boat is stranded, caught in a storm, out of port for longer than was expected) Marooned on an uncharted island
- Extended trips into the Underdark (We once ran a lengthy adventure through the underdark. Were down there for months! "If I never have to eat a fungus again...")
Every so often, if the party has NOT been somewhere to acquire supplies in a while (over...2 or 3 weeks, let's say) I'd ask how much food everyone had with them. But in general, yes, so long as they're somewhere where it is possible, I just handwave that they can find some rabbits or a couple of fish...with druids, rangers, elves, gnomes or halflings in the party, berries, fruits (in season, of course

and/or edible roots are assumed.
But again, unless it is "harsh" terrain with limited (or no) vegetation and sparce wildlife, we don't need to talk rations.
For shelter, again, only if there's some harsh environment.
However, shelter is also dependent on how closely you will be paying attention to things like seasons and weather patterns.
Heavy rains in the spring? Thunderstorms (or hurricanes near coasts) in late summer. Blizzards.
In these cases when I'd incorporate harsh weather, shelter becomes a challenge for the players.
It
is D&D, so most of the time there's always a small dry cave to be found (inhabited, of course...or found by something else seeking shelter from the storm while the party sets up camp

Or a "Dryish" patch in the woods.
I pretty much assume, if there's anyone with ANY kind of outdoor/survival skill (or even just an elf) in the party, that they are capable of making a lean-to, at least.
Available shelter is a matter of location.
In areas where travel is common, trade routes, etc...there may very well be a random inn along the road (in relatively secure lands) or at least some kind of unmanned travelers'/mountain refuge set up along the path.
The hiker's shelters I've seen are usually just a low open building (2 or 3 walls and a roof and a fire pit for cooking). 20 or 30 feet x 20 or 30 feet...large enough for a small group and a couple of mounts to be out of the direct elements. (Naturally, if you want to get creative, if you're in a mountainous region it could have been built by/for dwarves who used to traverse the area and thus only 5 or 6 feet high.
Naturally, any nasty creatures in the area would know of these shelters, possibly use it themselves in harsh weather or at least patrol it regularly to see if any easy prey stopped in.
Things like run-of-the-mill colds or disease I tended to stay away from...adventurers being the hardy sort that they are. I am reminded, however, of one player who suffered from terrible allergies in the spring and incorporated them into her character. It was rather fun(ny) to hear her speak all stuffed up and sniffling...the occasional sneeze.
Next time spring came around IN-game (it wasn't spring in real life) she started sneezing and talking all stopped-up again. It was really great. (and she was playing a rogue, so sneaking about became a bit challenging)
But that was the player's choice, not anything I imposed.
--Steel Dragons