Which viruses are the most dangerous?

They have shaped our evol­u­tion and can be prom­ising med­ic­al tools, but vir­uses are above all asso­ci­ated with the notion of dis­ease. They even take their name from the Lat­in word for pois­on… Let’s take a look at the most dan­ger­ous spe­ci­mens in the vir­al menagerie.

The first thing that comes to mind when we talk about how dan­ger­ous a vir­us is gen­er­ally its leth­al­ity: if a per­son is infec­ted, what is the risk of them dying? Some vir­uses are par­tic­u­larly wor­ry­ing from this point of view. The rabies vir­us, which affects the nervous sys­tem, is almost 100% fatal once symp­toms appear. The best defense against this dis­ease is vac­cin­a­tion. Use­ful before or after expos­ure to the vir­us, it can be used in humans but also in anim­als likely to con­tam­in­ate them, mainly domest­ic dogs¹. 

HIV, the Human Immun­ode­fi­ciency Vir­us, is also fatal in almost 100% of cases if left untreated. Less than one per­cent of patients seem to be able to spon­tan­eously con­trol the vir­us and avoid the devel­op­ment of Acquired Immun­ode­fi­ciency Syn­drome (AIDS)². For­tu­nately, cur­rent treat­ments can con­trol HIV so effect­ively that infec­ted people may no longer have symp­toms, no longer be con­ta­gious and no longer die because of the vir­al infec­tion. Des­pite the absence of a vac­cine, effect­ive pre­vent­ive approaches do exist, not­ably pre-expos­ure pro­phy­lax­is, or PreP. How­ever, access to these ther­apies remains unequal across the world, and there is no treat­ment that can be used on a large scale to cure HIV.

The examples of rabies and HIV show that the intrins­ic leth­al­ity of a vir­us can be greatly reduced when effect­ive means of pre­ven­tion or treat­ment are avail­able. Pro­gress is being made in these areas with anoth­er type of par­tic­u­larly leth­al vir­us: filovir­uses. This is the fam­ily of vir­uses that includes Ebola and Mar­burg, which do not cause dis­ease in some Afric­an bats but do cause hem­or­rhagic fevers in humans. The aver­age fatal­ity rate is around 50%, but var­ies accord­ing to epi­dem­ic and strain, and has already exceeded 80%³. The WHO now recom­mends two mono­clonal anti­body-based treat­ments for the most dan­ger­ous strain of Ebola⁴, as well as two vac­cines, although research is still under­way to determ­ine the best vac­cine regi­mens⁵. 

We can reduce leth­al­ity of vari­ous vir­uses thanks to vac­cines and treat­ments. But we must not for­get that they are extremely unevenly avail­able around the world, and vary accord­ing to geo­graph­ic­al area, geo­pol­it­ic­al situ­ation, or fin­an­cial resources. Not all pop­u­la­tions have the same oppor­tun­it­ies when faced with the same infec­tious agent⁶.

Some vir­uses become par­tic­u­larly dan­ger­ous when they trig­ger spe­cif­ic dis­eases: the severe form of yel­low fever or the pul­mon­ary syn­drome caused by cer­tain Hant­avir­uses, trans­mit­ted by rodents⁷, can exceed 50% leth­al­ity. For­tu­nately, how­ever, these clin­ic­al forms are rel­at­ively rare. This illus­trates anoth­er para­met­er to be con­sidered when assess­ing the danger of a vir­us: the num­ber of people it infects and makes ill. The Crimean-Congo hem­or­rhagic fever vir­us, trans­mit­ted by ticks, is a cause for con­cern, with a fatal­ity rate of around 40%. It is there­fore being mon­itored and stud­ied, but although it has been present for dec­ades and is endem­ic in some coun­tries, few­er than 20,000 cases have been recor­ded in total⁸.

To assess the real impact of a vir­us, it seems rel­ev­ant to look at the num­ber of deaths it has actu­ally caused. From this point of view, HIV, with its 40 mil­lion deaths over forty years, remains the main scourge of the present day, but SARS-CoV­‑2 has affected so many people and had so many indir­ect impacts that the Cov­id-19 pan­dem­ic may have caused more than 18 mil­lion deaths in just two years, i.e. around thir­teen times more than HIV over the same peri­od⁹. This con­sid­er­able num­ber is still lower than the con­sequences of the Span­ish flu, which is thought to have claimed at least 50 mil­lion lives in 1918 and 1919, in a con­text com­plic­ated by the First World War¹⁰. This pan­dem­ic is con­sidered to be one of the worst ever exper­i­enced by humanity.

How­ever, the fur­ther back we go, the more dif­fi­cult it is to make estim­ates, and com­par­is­ons lose their rel­ev­ance if we don’t con­sider the increase in the num­ber of humans liv­ing togeth­er on our plan­et. If we exclude plagues caused by bac­teria, the worst pan­dem­ic is prob­ably that caused by small­pox. With a mor­tal­ity rate of around 30%, major after-effects in some of the sur­viv­ors, thou­sands of years of propaga­tion and 300 mil­lion vic­tims in the twen­ti­eth cen­tury alone, it’s enough to make you shud­der¹². This dis­ease, brought to Amer­ica by the con­quista­dors, is even thought to have played a fun­da­ment­al role in the con­quest of the New World, because the nat­ive pop­u­la­tion had no immunity at all¹³. But small­pox is also one of human­ity’s greatest suc­cesses: since 1979, it is the only human dis­ease to have been offi­cially erad­ic­ated thanks to vaccination.

The mor­tal­ity rate of a vir­us is inev­it­ably a sig­ni­fic­ant factor, but it is far from the only para­met­er to be taken into account when assess­ing how dan­ger­ous it is. For example, the impact may be con­sidered more ser­i­ous when it affects cer­tain pop­u­la­tions, such as chil­dren. Rota­vir­uses, for example, do not appear to pose much of a threat if they are presen­ted as the cause of gastro-enter­it­is. But they par­tic­u­larly affect chil­dren under the age of five, in whom they can cause severe dehyd­ra­tion lead­ing to hos­pit­al­isa­tion. More than 180,000 chil­dren died from them in 2017, mainly in low- and middle-income coun­tries¹⁴.

Fur­ther­more, how dan­ger­ous a vir­us is depends very much on its rate and modes of trans­mis­sion. Measles is par­tic­u­larly impress­ive in this respect: one infec­ted per­son can infect around fif­teen oth­ers, mak­ing this dis­ease very dif­fi­cult to con­trol. It is so con­ta­gious that its spread can only be stopped if 95% of the pop­u­la­tion is immun­ised. How­ever, vac­cin­a­tion cov­er­age is far from reach­ing this level every­where in the world, includ­ing Europe¹⁵. As a res­ult, no coun­try has man­aged to rid itself of this deadly dis­ease, which remains a cause for con­cern for health agen­cies¹⁶.

The three coronavir­uses that have caused prob­lems over the last twenty years illus­trate the import­ance of con­ta­gious­ness in the danger posed by a vir­us. MERS-CoV, which appeared in Saudi Ara­bia in 2012, seems wor­ry­ing with its case-fatal­ity rate of around 35%, but it has caused few­er than 1,000 deaths in total because it is very dif­fi­cult to trans­mit between humans: most cases res­ult from con­tact with dromedar­ies car­ry­ing the vir­us¹⁸. Con­versely, SARS-CoV­‑2 has a fairly low case-fatal­ity rate (which is dif­fi­cult to estim­ate at the moment because it var­ies depend­ing on the vari­ant, age, level of immun­isa­tion, etc.), but it has caused many more deaths because it has infec­ted more people. 

For its part, SARS-CoV­‑1, which appeared in 2002 and was respons­ible for SARS (Severe Acute Res­pir­at­ory Syn­drome), was as con­ta­gious as SARS-CoV­‑2 at the start of the pan­dem­ic, and had sim­il­ar modes of trans­mis­sion. How­ever, this vir­us, which is leth­al in almost 10% of cases, was stopped in just a few months, where­as its more recent cous­in became uncon­trol­lable… Because car­ri­ers of SARS-CoV­‑1 were only con­ta­gious when they were symp­to­mat­ic. It was there­fore easy to set up effect­ive quar­ant­ines. SARS-CoV­‑2, on the oth­er hand, can be trans­mit­ted by people who show no symp­toms, mak­ing it much more dif­fi­cult to stop. The con­ta­gious­ness of asymp­to­mat­ic people is there­fore also a danger factor. It also partly explains the spread of HIV, where car­ri­ers can be con­tam­in­ated for up to ten years before devel­op­ing symptoms.

Finally, to assess how dan­ger­ous a vir­us is, we need to con­sider all its con­sequences, which, as the Cov­id-19 pan­dem­ic clearly showed, are not lim­ited to mor­tal­ity. Hos­pit­al­isa­tion, which can over­whelm a health­care sys­tem, and long-term after-effects, which have health implic­a­tions as well as social and eco­nom­ic ones, can also be sig­ni­fic­ant. This was the case with small­pox, which caused scars, par­tic­u­larly on the face, but could also lead to blind­ness. Polio, which has almost dis­ap­peared thanks to vac­cin­a­tion but is still cir­cu­lat­ing in Pakistan and Afgh­anistan, can cause per­man­ent para­lys­is¹⁹. And Lassa fever, endem­ic in West Africa, causes deaf­ness and myocardit­is²⁰. 

Papil­lo­mavir­uses can cause can­cer not only of the cer­vix, but also of the vagina, vulva, anus, penis and oral cav­ity. Vac­cin­a­tion against cer­tain strains can now drastic­ally reduce these risks.

The list of vir­uses whose con­sequences per­sist over time is a long one: we could add to it those that pro­mote the devel­op­ment of can­cers, such as papil­lo­mavir­uses or hep­at­it­is B and C vir­uses²¹, those likely to cause more severe symp­toms when react­iv­ated after the ori­gin­al infec­tion (such as the chick­en­pox vir­us, also respons­ible for shingles) or those which, ini­tially per­ceived as fairly harm­less, actu­ally appear to be linked to ser­i­ous ill­nesses. The most recent example is the Epstein-Barr vir­us, a herpes present in 90% of the pop­u­la­tion, which is clearly asso­ci­ated with the devel­op­ment of mul­tiple scler­osis²².

Once all these factors have been taken into account, it seems illus­ory to identi­fy THE most dan­ger­ous vir­us. But hav­ing them in mind enables us to know which vir­uses to keep a close eye on, and to con­sider ways of mak­ing each of them as safe as pos­sible, in par­tic­u­lar by abol­ish­ing the inequal­it­ies in access to treat­ment and pre­ven­tion tools that still divide the world. Obvi­ously, this debate must be exten­ded to non-vir­al infec­tious agents: bac­teria, fungi, and oth­er para­sites, such as the Plas­modi­um respons­ible for mal­aria. The One Health approach is also a remind­er that humans are part of eco­sys­tems and that health issues must be con­sidered on an envir­on­ment­al scale²³.