In about six months time every human on Earth is infected with a virus. How would a virus spread to cause such a quick transmission rate to infect 7+ billion people?
First, the virus has virtually no symptoms. If there are symptoms, we notice. If it does have any symptoms, they have to be sneaky, like increasing our sweat just a bit, or making a person slightly more needy of social contact. This probably means it hijacks and suppresses the immune system to some degree.
Second, the virus pretty much has to be airborne, shared by breathing. That’s the easiest transmission method. Sexually transmitted diseases just don’t get around (ironic). Skin contact diseases do spread, but not in casual interactions.
So, you’re looking at an airborne measels-meets-AIDS combo. Yuck.
But even with high communicability, you still have distribution problems to remote islands, Arctic and Antarctic climates, etc. I strongly doubt you can get every human within six months unless you have someone consciously spreading it. An organization playing courier will be needed to get it around that fast.
SRM covered a lot so i won't repeat the lack of Symptoms or the airborn part but i do want to expand on his
someone consciously spreading it
Ever heard of the Blood Plague Incident? IT was in the MMO World Of Warcraft where a "disease" got spread outside of where it was supposed to stay. This caused major community hubs to get infected fast with a high mortality rate. Problem is people wanted to avoid that so they went to more remote locations, with the virus following due to infected people trying to go there too (To intentionally spread it because of the "If i die we all die" mentality or just seeking refuge).
So that would be the reason remote places get infected, people fleeing or spreading out of s sense of "fairness". So for it to work he disease should be spreadable way before the it is detectable.
A likely scenario would be people fleeing infected areas, some of them are carriers (hell maybe even all of them) who think they are not sick and view the original inhabitant as selfish/paranoia and justify them taking control (and spreading the disease or at least kill the healthy) out a sense of need.
Major International Events
The first cases appear in a city shortly before it hosts the Olympics, World Cup, or a similarly-sized international event, and many infected people are working the event in positions with lots of public contact (ticket-takers, concessions, athlete wranglers, etc.) Fans from all over the world attend and pick up the disease. The fans spread it further as they journey home, including many long flights with stopovers at major international airports as well as trains, buses, etc. Athletes pick it up and, being very fit and healthy to start with, don't show symptoms while they do press tours, public meet-and-greets, parades, etc. upon returning home.
By the time anyone's showing symptoms a few weeks later, it's all over the globe and spreading rapidly.
Use birds as (additional) carriers.
As others have pointed out, some small pockets of human civilization might be completely isolated (though personally I doubt that they don’t have any (in)direct contact). But I’m pretty sure all of them have some contact with birds. The only possible exception is the Amundsen–Scott South Pole Station which is completely isolated for seven months between February and October. You’d have to time your virus correctly to infect them before the last flight in mid-February.
If you really aim at infecting
every human on Earth
that's highly unlikely, bordering the impossibility. There are human communities which are so far apart from other communities that the virus won't be able to reach them.
Think of the people living on Sentinel islands, or remote tribes in the Amazon forest, for example.
Our modern and high developed transport network makes it rather easy for a pathogen to quickly travel between continents, but then a capillary diffusion within the continent is another story.
It might be easy in Europe and North America, but for Asia, Africa South America, Oceania and polar regions it might take way longer than 6 months, if it can happen at all.
There is a theory that some viruses may originate from space (panspermia). So if you take the concept of a readily transmissible virus with minimal early symptoms (the "measles-meets-AIDS" virus suggested in other answers) and combine it with the idea of the Earth encountering a fairly broad cloud of virus-bearing material then you could start the infection process all over the place. Even small remote communities would see some initial patients, and the high transmissibility would account for spreading it to the rest.
I would probably not have suggested this answer if you had used the "hard-science" tag as while the panspermia hypothesis is credible enough for a significant number of experiments relating to it to have been carried out by NASA, ESA and maybe others, the results of those experiments appear to quite significantly constrain accidental panspermia, albeit not entirely rule it out. Essentially you need meteorites at least 1m in size to carry the micro-organisms in order for them to survive atmospheric re-entry. Therefore, for it to happen everywhere at once and not to have happened before you need something like a meteor storm but you can't rely on the existing annual meteor events. You probably need to invoke something like a new comet to dump a whole load of suitable rocks bearing the virus into Earth's orbital path. So it depends whether you want your plot to be fully "hard science" or whether you're happy with a "plausible but not likely in practice" explanation. Of course, depending on your plot you might have the option of deliberate panspermia, where the virus is seeded across the world by aliens.
You make it generally harmless, and spread through common animals that aren't seen as pests. Sounds like you want to read about Toxoplasmosis
Up to half of the world's population is infected by toxoplasmosis, but have no symptoms. In the United States, approximately 11% of people are infected, while in some areas of the world this is more than 60%. Approximately 200,000 cases of congenital toxoplasmosis occur a year.
The virus spreads rapidly. We ourselves spread it. If we are infected by a virus and we meet someone, we cause them that virus too. But sometimes it is not the virus in my view. It is the environment that is polluted. And pollution causes harmful viruses. And of course, it will infect, everyone living in that environment. An example is the Holi festival in India. On Holi, everyone plays with colors that are toxic. So the color rises up into the air and everyone breaths in it. So almost everyone is infected with some respiratory issues or throat infections due to the presence of the fatal virus in the air.
Thing is there are tens of thousands of medical facilities performing blood-tests every minute around the globe. The existence of the virus would be discovered early on and people would start contingency operations. Bunkers, quarantine facilities, etc. People are resilient and inventive.
The only way this could work would be the collapse of the civilization as we know it before the virus dissemination is started. The whole of the humanity going back to middle-ages, but even then you can't rule out bubbles of the old society living isolated underground or in hidden areas, some of them military protected and others going to great pains to remain hidden and unknown. There are people right now concerned with this exact scenario you are trying to concoct. I'm talking survivalist, be-prepared groups active on message boards, with tons of information regarding how to outfit your bunker best :) The downside of the civilization collapse would be that you'd have fewer ways to spread the virus (no planes, trains, cars, mails).
So you'd have to operate from space and not rely on the existing infrastructure. You'd have to have access to vast amounts of resources, deploy pods from outer space in a systematic manner.
And still I don't think it's possible to infect the whole of the humanity in its entirety.
Leaving aside the difficulty of reaching purposefully isolated and hidden groups living in bunkers or other hidden facilities, individual humans are unique at genetic level. There would still be people immune to the virus. You just couldn't get them all sick.
I am not a specialist but heard about a real virus that seems to achieve what you want: It's name is "Papillomavirus".
It is a virus transmitted through genital zones, as well as through mouth. Thus, it is supposed to have infected every men and women in developed countries after 25 years. The thing is that you don't have symptoms, and no directly related diseases as well for most people, so the virus is not detected.
You can have the disease actually change people's behavior so they want to get away anyone they know and towards people they don't know. This is similar in concept to how the parasitic wasp infects caterpillars- caterpillars protect the wasps (having been inside and eating and affecting the caterpillar) that burst from their skin with their own silk and attack any creature getting near the wasps.