The plural form of electron is electrons.

What is the plural of helmet?

helmet /ˈhɛlmət/ noun. plural helmets.

What is the correct plural of virus?

As a Latin word, 'virus' does not have an attested plural. It has a second declension genitive singular but it is quite rare. The word isn't common in general. As a result, the appropriate English plural really should be 'viruses'.