On first glance, you might think that a cold is just a mild flu, and fact this might be the case 15-20% of the time due to the fact that they share many symptoms and even look similar on the molecular level (not genetically, though).
Although both are viruses that fly through the air in an almost spherical capsid, the common cold (Rhinovirus) doesn’t cause nausea, diarrhea, or vomiting. The restriction of the virus to the lungs is because unlike influenza, rhinovirus remain in the upper respiratory system (so the nose and nasal cavities). Rhinovirus prefer slightly cooler temperatures than influenza viruses, which is part of the reason it doesn’t infect other tissues.
Alongside being a little bit smaller – Rhinoviruses are around 30 nanometers in diameter, compared to 50-120 nm for influenza- and preferring slightly lower temperatures (which is part of what keeps Rhinovirus from infected other tissues), neither Rhinovirus or Adenovirus (about 5% of “common cold” infections) will kill even an immunocompromised person in most situations. This cannot be said about influenza, of which there are several types.
Influenza is a family consisting of 3 genuses (A, B, and C), which the latter two affected primarily humans (type B also infected seals and type C pigs ) and type A affected a wide range of hosts including birds and thus presents us with the most dangerous influenza strains around. Influenza A is the most dangerous because humans have trouble adapting due to their inter-species mutations. The most deadly influenza of all time, the 1918 flu, was actually one of the most avian of human influenzas and is a common ancestor of many modern swine and avian influenza viruses.
The fact remains that human life in civilization has granted most of us increased resistance to both the flu and the common cold. These resistances are not shared by communities who have never been in contact with civilization, often leading to high death rates when an infected foreigner comes into contact with an otherwise isolated group. Despite this resistance, the chance of an equally catastrophic influenza outbreak as that seen in 1918 is not particularly low.