A vector transmits a disease or parasite from one animal or plant to another. Fruit bats are vectors for Ebola. They are a host to the virus, which can then be passed on to humans.
Influenza A, the virus responsible for human seasonal epidemics, is not a vector for other pathogens, although co-infection with two different strains of influenza can give rise to progeny viruses with mixtures of genes derived from the parental strains. If anything, that would make humans, other mammals and birds the vectors for Influenza A.
Influenza A doesn't carry plasmids, and it does not insert it's genes into cell DNA as does a retrovirus.
The most frequent serious complications of influenza are pulmonary and fall into 4 categories or groups:
- primary influenza pneumonia
- secondary bacterial pneumonia (also called co-infection, but from different sources)
- pneumonia due to unusual pathogens or in immunocompromised hosts
- exacerbations of chronic pulmonary diseases
Typically, the first group is smaller than the second (except for Avian Influenza A). No additional infection is necessary in this group to cause pneumonia or death.
In the second (and largest) group, typically viral influenza infection is followed by near resolution of symptoms, subsequently complicated 4 to 14 days later by a recurrence of fever, dyspnea, productive cough, and pulmonary consolidation. Common pathogens are S. pneumoniae, S. aureus (methicillin-sensitive [MSSA] or methicillin-resistant [MSSA]), S. pyogenes, H. influenzae, and some others.
The pathogenetic mechanisms of viral-bacterial co-infections result from virus induced alteration in pulmonary epithelial cells (e.g. reduced ciliary function, cell death/decreased epithelial barrier function, etc.), virus-mediated inhibition of innate immune cells (e.g., suppressed phagocytosis, impaired microbial killing, etc.), dysregulated inflammation (e.g. increased inflammation due to chemokines) and other mechanisms.
Pneumonia due to unusual pathogens or in immunocompromised hosts, and exacerbations of chronic pulmonary diseases are less common than the second group but carry a high mortality.
The second reference is an exhaustive review which you might find quite helpful.
1 FluGenome: a web tool for genotyping influenza A virus
2 Complications of Viral Influenza
3 The co-pathogenesis of influenza viruses with bacteria in the lung