Because of the long onset..

That is meaningless, easy to say and doesn't bring anything new. Could you please elaborate?

any way TB is serious 

I'm not an expert on this, though maybe it has to do with the smaller size of the virus with large numbers of infective particles per droplet , the fact that new viral variants arise often with the higher likelihood of a high proportion of people being successfully infected?

Viruses require living hosts — such as people, plants or animals — to multiply.

Bacteria are relatively complex cell.  They can reproduce on their own. Fossilized records show that bacteria have existed for about 3.5 billion years, and bacteria can survive in different environments, including extreme heat and cold, radioactive waste, and the human body. Can we say human body knows TB bacteria, human body was introduced to BCG vaccination. Now we have a different living condition. In order for  flu virus to be effective it has to have wright host and it has to mutate. When I think about developing world, when I think about poor countries devastated by war and famine, TB rates are going up, and other  infections like Highly pathogenic Avian influenza H5N6, Chikungunya, dengue etc….So this is maybe a research question.  I still don’t have an answer why TB rates are bigger in refugees than among other foreign born persons-so that is again research question.

Good question.  To my way of thinking, the main difference is likely the sheer numbers of virions that are shed when a person with influenza coughs/sneezes.  It is also fairly effectively spread by hands when they are contaminated with mucus that contains the virus.  

Everybody with symptomatic influenza virus infection is shedding the virus in large quantities; many people with active TB are shedding only very small amounts of the bacterium.  Sometimes we can't even find it in sputum smears even in people we know have active pulmonary disease.  

The immune system handles the organisms very differently undoubtedly accounting for much of the phenomena in the question.   The vast majority of those infected with active influenza disease will self cure.  Only the minority those with active TB will self cure, although the large majority of those infected (but not with active disease) will contain the infection, but will not eradicate the organism.  On the other hand, boosting the immune system through vaccination with influenza is "relatively straightforward" , whereas effective TB vaccination has proven to be exceedingly difficult.  The fact that spontaneous mutations occur frequently with influenza and very infrequently with the TB organism is perhaps significant.

Dear all, so far we have had some very good answers to this question. By raising this question I would like to have ideas and views on what is limiting the transmission of TB compared to other airborne pathogens. What mechanisms are acting to shape this mode of transmission? Surely host as well as pathogen factors play a role in transmission, but what host and pathogen mechanisms define transmission of TB compared to other airborne pathogens such as influenza?

David, thank you for your comments about new viral variants and large numbers of infective particles per droplet. Also, Randall thank you for your comment on the numbers as well. Surely the large numbers are an important factor promoting infection. However, in a close room, for example in a class room, if one person has active TB and stays long enough contacting other people, only a few will be infected. However, if this person is infected with an influenza virus, most contacts would be infected. I think the problem is not only related to numbers but also to the mechanisms of transmission and establishment of infection, both dependent on the host and pathogen.

Melvin, thank you very much for your comments. Your points are very important. The pathogens in question are very different indeed, however, my point in comparing two different airborne pathogens one of which infects most individuals (influenza) while the other infects one third of the population (TB), is to raise the question of the transmission of TB being different comparing to other airborne pathogens. Concerning new vaccines being developed in the last 15 years all around the world currently in clinical trials to improve or to complement BCG, there have been several degrees of success, but they all still fail to prevent transmission. My point is that the transmission of TB is still very puzzling, when we look at the outcomes of the infection, compared to other airborne pathogens.  As you mentioned, “the immune system handles the organisms very differently undoubtedly accounting for much of the phenomena in the question”.  The combination of these host mechanisms with pathogen factors will define the nature of the TB infectivity. This is the question that I am addressing here and that I want to understand, to help finding a solution for this problem.

Recently, I published a study where I tested the infectivity of different species of mycobacteria in different macrophage cell lines and primary cells. The results suggested that the higher the virulence of a mycobacteria species, the lower its rate of infection in human monocyte derived macrophages (see link below). I am exploring this avenue to understand the host and pathogen mechanisms that affect transmission and establishment of infection.

Please keep on following this question and suggest new ideas on how to address this problem.

Chapter Mycobacterial infection of macrophages: the effect of the mu...

TB (bacterian and chronic infection) and Influenza (viral and epidemic infecion) are two diseases which differ on the epidemioloical pathway. TB is less contagious than Influenza.

Each disease distribution and their severity variant from one to another depend on causative agents features ( resistance to environmental factors )available , suscesptible host and the the epidemiological factors which limit the pathway of disease to be sever contagious or less .

There are three relevant factors: the nature of the infecting organism; the state of the immune system; the nature of the channels by which organisms spread throughout the body. TB can lurk for a long time inside the lymphatic system, and in niches in the body outside the influence of antibodies.

Size of droplets, innate immunity!!