Freshers' Flu is a psycho-socially advanced poly-virus, experienced in some form by around 90% of all new university students during the first few weeks of term.
In many ways it is related to the form of cold/flu bug(s) suffered by a lot of children starting at school, their parents, their siblings, and teachers starting at a new school, but it has many features of its own which are only very rarely present in these related illnesses.
The illness can be seen to be made up of three parts: the viral, the psychological, and the social.
The viral part is that part which is shared with the other scenarios listed above, and consists basically of a mixture of standard cold and flu-type viruses from different areas, which are brought into contact as a mass onslaught to the immune system of the person in question. This normally leads to at least some holes being found in the immunal defences which have so far been built up by the individual's experience. Here, however, lies the first distinguishing feature of Freshers' Flu as compared to other related illnesses: its "catchment area". Unlike a simple school situation, the constituents of Freshers' Flu are likely to come from around the entire country (and, in some cases, the world), as the victim mixes with such a wide variety of people in an attempt to make new friends. This greatly increases the chance of catching the illness, and its subsequent seriousness.
The second part is the psychological part of the illness. While one element of this, in terms of the stress of new experience, is common to all scenarios, there is an added element in the case of university students. This is that their immune defences are further weakened by an extra layer of stress caused by having "left home", with all the consequences of that independence, such as washing, cooking and attempting to be organised of one's own accord - quite apart from varying levels of homesickness...
The weakness induced by the psychological part of the illness is further increased by the social part. This is caused by the need to make a lot of new friends, adapt to a new life, and not seem boring (to others or to yourself). There is a definite tendency for the first few weeks of university to be intentionally turned into a blur to try and minimise the psychological problems outlined above. The effect is, if not the opposite, another side of the same coin: the combined weakening effects of alcohol and other drugs1 consumed in larger quantities than previously and a succession of late nights and long walks to and fro in insufficient clothing.2
In summary, the combined effects of all of the above lead to most people walking around campus in the first few weeks of their first term at university extremely tired and/or sleepy and/or coughing and/or nursing a very sore throat - or in extreme cases not walking around campus at all (or not at the level they should be). Sadly, neither viruses, homesickness or psychological adjustment can currently be cured, and socialising is an intrinsic and formative part of many people's university experiences, so Freshers' Flu is probably here to stay.
1 tobacco, for instance
2 It is a common, although not universal, problem for freshers to forget that the weather will turn cold very soon, and that if they do not bring a winter coat with them, they will be cold.