ail

contagion, contagious disease

contagium, infection

attack, bout, fit, spell

debility, decrepitude, feebleness, frailness, lameness, sickliness, unhealthiness, unsoundness, unwellness, weakness

malaise, matter, pip

epidemic, pest, pestilence, plague

aggravate, anger, annoy, bug, chafe, chivy (or chivvy), exasperate, fret, gall, get, grate, harass, harry, irk, irritate, nettle, peeve, pester, pique, put off, put out, rile, vex

bedevil, haunt, plague

abash, confound, confuse, discomfit, disconcert, discountenance, embarrass, faze, fluster, jar, mortify, nonplus, rattle, shake up

daunt, demoralize, discourage, dishearten, dispirit, unnerve