Antivirals: targeting a hijacker
Viruses are the hardest target of all, because they replicate inside your own cells using your own machinery. There is very little that is uniquely 'viral' to attack — so selective toxicity is harder to achieve here than anywhere in antibacterial therapy. The trick is to find the few enzymes a virus brings of its own. An antiviral like aciclovir, for instance, only blocks viral DNA replication and is activated only inside infected cells, sparing healthy ones.
Aciclovir is a beautiful example of a prodrug: it arrives inactive and is switched on by a viral enzyme that healthy cells lack. That extra layer of selectivity is exactly what makes some antivirals tolerable. Modern antiviral regimens — for HIV or hepatitis C — combine several drugs hitting different viral steps, the same combination logic used against resistance in earlier guides.
Antifungals: a thin line of difference
Fungi are true cells like ours, with a nucleus and similar machinery, so the safe differences are few. The classic antifungal target is ergosterol, the sterol that fungi use in their membranes where we use cholesterol. Azoles block its synthesis; polyenes like amphotericin bind it directly. The trouble is that these drugs still nudge our cholesterol-rich membranes, which is why amphotericin is famously hard on the kidneys — a vivid reminder that weaker selectivity means more toxicity.
Antiprotozoals and the unifying lesson
Protozoa — the parasites behind malaria, giardiasis and others — are complex cells with their own quirks to exploit. Antimalarial antiprotozoals target the parasite's unique handling of haem inside the red blood cell, or pathways it cannot do without. Because parasite life cycles have several stages, treatment may need different drugs for different stages, and travellers often take chemoprophylaxis before entering malaria zones.