Subjects must agree in number with their verbs. Most writers match nouns and verbs without difficulty.
Correct: Hunger in developing countries harms everyone.
Correct: Hunger and disease in developing countries harm everyone.
Correct: Hunger or disease in developing countries harms everyone.
More problematic are singular pronouns that some writers believe to be plural.
Singular: Everybody, everyone, everything.
Singular: Anybody, anyone, anything.
Singular: Somebody, someone, something.
Singular: Nobody, No one, Nothing.