If you venture to enough interesting destinations (and even some boring ones) you'll eventually have to worry about mosquito-borne diseases, but long-term travel can prohibit the use of prophylactics. In Africa I took the notorious Lariam (mefloquine) and was treated to weeks of troubled dreams. A particularly memorable one involved being pursued by a robotic, Terminator-style nurse figure that I repeatedly stabbed in the face until I awoke in a sweat, only to discover my girlfriend dead beside me and a bloody toothbrush in my hand. (Okay, that last bit wasn't true but the dream sure as hell was.)
Then one morning I took my pill like a good lad and was stricken with a severe indigestion that soon burgeoned into nausea and bile reflux. Hiking through the stunning Ethiopian highlands is less pleasant with what feels like a golf ball lodged in your crop. The following week this discomfort increased to include bloody vomit. That was my last tablet, and as soon as we reached Addis Ababa we invested in a net clearly designed for a four-poster bed. Due to the structural wire hoop it packed down to roughly the size of a 26" Alex Rims DC19 mountain bike wheel - not that convenient to carry in a backpack.
Many kind organisations in Africa distribute free mozzie nets to combat the continet's #1 killer, but the locals often reinvent these life-saving gifts as fishing nets. They land a bigger catch while their children still die. Consequently, most cheap guesthouses leave the intrepid traveller totally unprotected. Any netting on offer is usually rent with holes big enough to allow access to a tiger ("A tiger? In Africa?" - thank you Monty Python), the tying off of which drastically reduces the net volume, or the net net volume if you like.
Long after it would have been useful, I bought a good-quality, compact net by Australian company Equip. It's roomy enough for two but light enough to carry easily, and is treated with Permanet - an insecticide treatment which releases slowly over time so that the protection lasts pretty much forever. Treating the net prevents the little bastards from sticking you through the mesh should you accidentally brush up against it in your slumber. Pure-breathers, however, may not fancy the idea of sleeping in a chemical enclosure.
The fattest, hardest, most vicious breed of bloodsuckers for whom it has been my displeasure to be supper are the mosquitos of the Amazon basin. I once spent a night in the Colombian Jungle near the tri-border with Peru and Brazil, and what a hellish night it was. We were equipped with hammock-specific nets which secured fast around the tensioned ropes and hung down to the forest floor. A reasonable design except that mine seemed to immediately resound with that portentous high-pitched whining noise. Fair enough, I thought, a few will have crept in before I sealed up, except that it was more than just a few - it was hundreds of them and their probosces were as sharp as hypodermics thrust by syringe-wielding, bank-robbing junkies. They penetrated any clothing I had; they feasted on my back through the hammock; they stabbed my eyelids. I swiped and slapped and clapped until my hands were a gory mess. How the hell were they getting in? It wasn't until later that I discovered they rest in the leaf litter by day. When they emerged at dusk they were trapped inside my cage of death. In the tepid jungle I swathed myself in as many layers as I could find to keep their stings at bay: I wore socks on my hands; I wrapped a t-shirt around my head; the only thing their mandibles couldn't penetrate was the plastic of my poncho and rubber boots. I lay awake, wrapped in my nylon swaddling for 8 hours, hugging my legs. Beware the Amazonian parasites.