The UK National Security Capability Review of 2018 and the UK National Risk Register for Civil Emergencies (UKNRR) both identify disease as a serious and likely threat to national security.
For example the UKNRR[2:p34] suggests that pandemic influenza could result in 2,000 to 750,000 fatalities and "high levels of absence from work". It also notes that the 1918 Spanish Flu epidemic is believe to have been responsible for over 50 million deaths globally. Influenza is a virus disease and therefore not treatable by antibiotics, although it may be preventable by vaccination if an effective vaccine can be found for the current strain.
Influenza mutates into new strains rapidly, requiring new vaccines to be developed for each new strain and there is great concern amongst experts that influenza can cross with viral diseases in animals resulting in very virulent new forms. These forms may be of a kind for which there is little residual resistance in the human population and for which vaccine production may be difficult. Examples of these are 'swine flu' (H1N1 strain of virus) which killed 18,500 people in 2009 and the H5N1 strain known as bird or avian influenza which killed several hundred people in South East Asia in 1996[2:p34]. The relatively small number of deaths from the H5N1 strain is attributed not to low lethality but to its relatively poor human-to-human infection rate and this may not always be the case as noted in an article in the Malaysian Journal of Medical Science which starts:
"The World Health Organization (WHO) and influenza experts worldwide are concerned that the recent appearance and widespread distribution of an avian influenza virus, influenza A/H5N1, has the potential to ignite the next influenza pandemic. It is estimated that even in the best-case scenarios, the pandemic will cause 2 to 7 million human death and tens of millions would require medical attention. If the pandemic virus is a very virulent strain, deaths could be dramatically higher."
As Dr Harvey Rubin, University of Pennsylvania remarks "In 1957, an H2N2 pandemic and in 1968 an H3N2 pandemic caused approximately two million deaths and one million deaths respectively worldwide".
The UKNRR quotes four to six months as the time to develop an influenza vaccine for a new strain. An article published by CNN suggests that it may be possible to reduce the time taken to develop a vaccine from 30 weeks to 6 weeks and reduce the severity of an epidemic of a normal strain of influenza, but also points out that it took 12 years to develop a vaccine for Ebola, a non-influenza virus which was first identified in 1976 and has 25%-90% mortality, spreading rapidly by contact with infected body fluids.
New viral (and other) diseases appear from time to time. The UKNRR points out that
"Over the past 25 years more than 30 new (or newly recognised) emerging infectious diseases have been identified around the world, such as Ebola, Zika and Middle East Respiratory Syndrome. The latter emerged recently in 2012 and poses a global health threat."
Rubin[6:p55] mentions SARS as a previously unrecognized virus:
"The Severe Acute Respiratory Syndrome (SARS) started in the Guandong province of China and enveloped the world by 2003. The infectious organism, later determined to be a previously unrecognized coronavirus, is a non-segmented, positive strand RNA virus. Approximately 9,000 individuals were infected, with very high mortality rate."
Viral diseases are a particular current threat due to the long time taken to develop effective vaccines. For example, HIV was first identified as an epidemic disease in 1981 although it is thought to have first transferred from apes to humans around 1920. HIV mutates rapidly and effective vaccines remain unavailable in 2018 almost 40 years after it was first recognised. Deaths attributable to HIV are estimated at 25 million between 1981 and 2018.
Smallpox is a viral disease with a mortality of between 30% and 90% depending on the type of the infection (some forms are 100% fatal). The modern concept of vaccination derives directly from early work on Smallpox prevention, a procedure so successful that the World Health Assembly declared Smallpox eradicated on 8th May 1980, after heroic efforts which are nowadays perhaps under-appreciated. The disease is thought to have emerged from African rodents some tens of thousands of years ago. Live samples of the virus still exist in laboratories in the USA and Russia leading to concern about the purpose of retaining such a dangerous virus and speculation about the motives as to why and concern that it might at some point be used as a biological weapon.
Viral diseases are not the only threat. Although enormous success was achieved at combating bacterial disease during the 20th century, starting with the discovery of Arsphenamine in 1907 followed later by Penicillin and numerous other antibiotics, bacteria are now evolving resistance to existing antibiotics and it is proving difficult to develop new methods of defence. Bacterial diseases can be catastrophic. The bacterium yersinia pestis killed at least one-third of the entire European population between 1347 and 1353 during the plague known as The Black Death. Although currently treatable with antibiotics the bacterium still exists with outbreaks of plague continuing to occur in the 21st century. Antibiotic-resistant versions of the bacterium are now being detected and pose a significant health risk according to a paper published in 2007.
Other long-time scourges of humanity have, at least until recently, been treatable with drugs: amongst them notably Cholera and Malaria. Other lesser diseases like Dengue and Yellow Fever (amongst others) are spread like Malaria by mosquitoes but have the potential to evolve other mechanisms of spread or to suddenly develop more virulent strains.
A report on the pipeline of new antibacterial drugs was published by the Infectious Diseases Society of American in August 2018 and suggests that there is no room for complacency, describing some bacteria which are now almost entirely drug-resistant.
There is a wide range of potential impacts that an epidemic could have. Epidemics are by their nature fairly rapid in onset and geographically widely distributed, so if significant in effect, liable to overwhelm any national or regional response efforts. One could reasonably expect hospitals to be unable to cope with large numbers of seriously ill patients and resources such as drugs, drips, transfusion fluids and similar items to be rapidly exhausted, the exhaustion compounded if large numbers of people are too unwell to work in the replenishment supply chain.
The problems in supply chains should not be underestimated. In the absence of hospital facilities, one seriously sick person is liable to tie up at least one other adult as a carer, thus taking two people out of the available labour supply. Whilst in the short term concerns about survival far outweigh economic effect, a lack of skilled talent is likely to lead to rapid deterioration in the performance of critical infrastructure components.
The 2006 Cranfield/DEFRA report into resilience in the food supply chain[25:p111] goes into some detail about planning and preparation for potential epidemics from the perspective of individual companies operating in that sector. There is little evidence in the analysis that there is any overarching strategic planning in place from government to deal with such an eventuality.
Sandia National Laboratories in the USA performed an analysis of the expected effect on the economy of the USA of a typical influenza epidemic, concluding that the impact on GDP would be of the order of a maximum of approximately 3%, or $290 billion, predicated on a mortality rate of less than half of one percent.
A report published by the US National Institutes of Health on Influenza states an unusually high mortality rate of 2.5% for the 1918 epidemic in the USA:
"... the 1918 pandemic differed from other pandemics in several important clinical and epidemiologic aspects. Although the clinical course was usually self-limited, a substantially higher percentage of cases developed severe pneumonic complications. As a result, the case mortality rate in the United States averaged 2.5%"
This is attributed to pneumonic complications for which there would not have been drug therapies at the time. It seems reasonable not to expect such a high mortality rate should a similar outbreak occur in the developed world unless there is a simultaneous shortage of antibiotics or a concurrent collapse in healthcare delivery.
The regular influenza epidemics that we are accustomed to tend to cause inconvenience and inevitably lead a small number of fatalities in unfortunate individuals but normally pass without notice or extensive comment. A common feature of most years, they have very little effect on routine daily life. The general population has become accustomed to this and is possibly complacent about it, since epidemiologists evidently treat the threat seriously (leading to its prominence on the UKNRR).
The reason for concern amongst medical experts is that they are well aware that not only are there variants of influenza around which could prove to have both high fatality and high transmissibility, there are also new viruses appearing (like, say SARS or Nipah)  and also unknown viruses potentially lurking in animal reservoirs which could be equally deadly. The latter is referred to by the World Health Organisation as disease 'X' since they don't know what it will be or from whence it will appear.
There is evident concern in the healthcare community that the world is not strategically prepared for a high mortality pandemic of a new disease , coupled with an expectation that the probability of this situation arising is a matter of when, not if, even if nobody knows how intense it might be.
It's self-evident that a pandemic with a mortality of even 10% (uncontrolled SARS for example) would deliver a shock to the modern world of a kind that hasn't been seen for centuries. If it should approach the mortality levels of the Black Death or worse, the world will be transformed, potentially for ever.
What is likely to turn a 'mere' health crisis into something much worse is the system-of-systems dependencies in developed societies. At which point will the unavailability (due to illness or death) of the key experts who keep the power on and the networks operating tip over into mutual cascading failure? 30% absenteeism or 60%? It seems that nobody knows, and as this site tries to illustrate, when that happens, the situation suddenly becomes very grave indeed, quite possibly turning an emergency into a catastrophe.