Etymonline has this to say:
-ics
in the names of sciences or disciplines (acoustics, aerobics, economics, etc.) it represents a 16c. revival of the classical custom of using the neuter plural of adjectives with -ikos (see -ic) to mean "matters relevant to" and also as the titles of treatises about them. Subject matters that acquired their names in English before c.1500, however, tend to remain in singular (e.g. arithmetic, logic).
So yes, at some point in history, there were such things as physic (meaning "natural science"), mathematic (meaning "mathematical science"), etc. that were later turned into plural forms but kept being treated as singular.
Edit: having looked in a few more places, it appears that in contemporary English, it still makes some sense to have both the suffix -ic and its plural form -ics. According to the Collins English Dictionary, the former has kind of specialized in forming adjectives, while the latter is happily forming nouns:
-ic
suffix forming adjectives
- of, relating to, or resembling: allergic, Germanic, periodic. See also -ical.
[...]
[from Latin -icus or Greek -ikos; -ic also occurs in nouns that represent a substantive use of adjectives (magic) and in nouns borrowed directly from Latin or Greek (critic, music)]
[...]
-ics
suffix forming nouns (functioning as singular)
- indicating a science, art, or matters relating to a particular subject: aeronautics, politics
- indicating certain activities or practices: acrobatics
[plural of -ic, representing Latin -ica, from Greek -ika, as in mathēmatika mathematics]
The key here is that they are not just two unrelated suffixes. Much rather, one is etymologically a plural form of the other. As the American Heritage Dictionary succinctly puts it, -ics is "-ic + -s".
This is all down to the fact that English is a language of acutely mongrel lineage. It has substantive roots in Celtic, Romance and Germanic languages (to name a few) and a grammar that lends itself well to the adoption of "loan words" (non-native words adopted into the native tongue.)
The "standard" means of pluralising a noun is to append -s, with some conventional variations (eg -f becomes -ves, -y becomes -ies) for convenience in spelling and pronunciation.
However latin-based words tend to pluralise in the latin fashion, so for example bacterium becomes bacteria, and cactus becomes cacti. Similarly greek-based words will adopt the equivalent pluralisation appropriate for the original root.
Still other words of Saxon or earlier origin have lovely, earthy plurals that defy the "conventions" due to their traditional forms being maintained. Geese, Mice and Children owe their unusual conjugations to their ancient roots, and to the fact that they are common words whose everyday repetition keeps them from slipping into bland conformity.
In my experience, words which do not pluralise are those which relate to herding, hunting and the counting of animals. These words tend to be saxon (germanic) or celtic in origin owing to the presence of farming and hunting in Britain long before the Norman invasion. This can be inferred by the fact that sheep, cattle and game do not pluralise, while whales, sparrows and elephants (seldom hunted or farmed in Britain!) definitely do.
I suspect these tend to be a contraction of the traditional counting forms for such cases ("head" of cattle, "brace" of partridge, "shoal" of fish) but this doesn't really answer the question of why such plurals take the same form as the singular. It could be that when counted in such a way, the animals being counted were considered an uncountable, continuous quantity (similar to water or money) that could only be "counted" when quantified with their associated counter, so cattle would be rendered an uncountable noun by its quantifying counter head. It's interesting however to note that bird pluralises to birds, while aircraft does not pluralise.
Sadly for the non-native speaker, this makes learning the "rules" of English an arbitrary and frustrating affair. However, spare a thought for the Japanese, who do not have plurals for any but a few unique nouns, and must instead learn a separate counting-suffix and corresponding character (kanji) for almost every class of noun imagineable. There are in fact entire volumes of the things, and it would be nigh-on impossible for any person to learn them all. Wikipedia lists a choice selection.
Best Answer
td,dr: Just use English rules — unless nobody else does. Then do whatever they do.
Perhaps the best rule is to ignore the language of origin that we pinched the newly borrowed word from and just use the same basic English rules for all inflections (usually noun plurals), as though it were fully naturalized into English. It’s certainly the easiest rule to remember, since you don’t have to learn the inflectional morphology of dozens of donor languages that way.
The truth is that it depends in part on how well assimilated the word is into English, and what the tradition is in the particular domain where that word is being used. Sometimes more than one version of a plural inflexion exists, such as both the English indexes and the Latin indices for the noun index, where mathematics tends to use the more conservative form.
Greek and Latin loanwords used in science and other technical disciplines often conserve their classical plurals. Some are now relatively common words like species and series, which due to their Latin origin (and perhaps also other phonologic factors serve as both the singular and the plural of each. So too does French chassis, although at least that one has a pronunciation difference between one chassis and two chassis.
Other terms with non-English plurals whose special Latin or Greek plural forms may not always be observed outside of technical writing include such singular/plural pairs as genus/genera, antenna/antennae, vertebra/vertebrae, mare/maria, thesis/theses, crisis/crises, emphasis/emphases, taxon/taxa, mythos/mythoi, dictum/dicta, paramecium/paramecia, phenomenon/phenomena, penis/penes, clitoris/clitorides, crysalis/crysalides, octopus/octopodes, fungus/fungi, nucleus/nuclei, radius/radii, criterion/criteria, matrix/matrices, cicatrix/cicatrices, larynx/larynges, stigma/stigmata — and many, many more.
Some exotic inflections seem more resistant to naturalization than others, such as with crisis > crises and emphasis > emphases. I don’t know why that should be. Loanwords that start out with classical inflections often lose them as we become more used to them. For example, the OED observes that specimina was fairly common as the plural of specimen during the latter half of the 17th century. However, the “special” plural form specimina instead of specimens is now so vanishingly small as to be virtually non-existent; you now only find it, if ever, in highly technical treatises on biology or typography.
You also sometimes see special plurals from French like plateaux, châteaux, and tableaux, or from Italian like mafiosi, tempi, or paparazzi. For whatever reason, non-English plurals for words coming from languages other than Latin or Greek don’t tend to last as long as the truly classical ones sometimes do. My own theory is that this is because compared to your average joe, writers in the sciences are not only more “precisionist” in their writing, they also are more likely to have studied Latin (and sometimes Greek) in their own studies.
It just depends on the field, though. For example, in computational linguistics the word corpus comes up all the time, meaning a body or collection of texts. In that domain, the expected form of the plural is the Latin one, so you have several corpora. However, we do not (to my knowledge) use the Latin genitives unless we are forming the species name of a particular taxon. So the possessive of the singular corpus is simply corpus’s in English instead of the Latin genitive singular corporis, and the plural possessive is corpora’s in English instead of the Latin genitive plural corporum. (Again, taxa are excepted, but those aren’t really English away; they’re “modern” Latin.)
Now and then you find some imported German nouns retain their German plurals for a little while, depending on the field. John Maynard Keynes actually used Spielräume for a particular philosophical term, being the German plural of Speilraum. Or when talking about the semi-autonomous local governmental units of Germany or Austria, each of which is a Land, you may see English writers using Länder for several of these. But these are markedly foreign; they are unassimilated imports. You can tell a German noun is on the road to being more fully Englished as soon as it loses its initial capital, as we see people writing zeitgeist in English instead of Zeitgeist. If that needed to be made plural, I would not be surprised to encounter the uncapitalized version to be the Englished zeitgeists, as it stands out rather less than writing Zeitgeister would.
The OED attests hundreds of different singular-to-plural irregulars for imports, but most of these don’t last long in English. It might have been bruschetta/bruschette and pannino/pannini right at the start, but those didn’t lost long, such now people look at you funny if you speak of “a graffito” in the singular, and it is not uncommon to get “double” plurals, like “two panninis”.
Just use English rules on loanwords’ plurals — unless you’re working with one that is expected to retain its irregular form. Those you should be able to find in a dictionary.
And the plural of virus is viruses. Always. :)