Final answer:
Chemotherapy for infectious diseases is challenging because drugs must selectively target the pathogens without harming human cells. Fungi, protozoa, and helminths are eukaryotic and similar to human cells, making selective toxicity difficult. Viruses replicate within human cells, making it even harder to target them without causing harm to the host.
Step-by-step explanation:
Problems of Chemotherapy for Infectious Diseases
Chemotherapy is a term that refers to the use of chemicals, or drugs, to treat diseases including infections caused by various types of microorganisms such as viruses, fungi, protozoans, and helminths. One of the main problems of chemotherapy in treating these infections is the difficulty in targeting the infectious agent without also damaging the host's cells. This is particularly challenging with eukaryotic organisms like fungi, protozoans, and helminths because their cells resemble human cells. Antifungal drugs, for instance, target the synthesis or function of ergosterol, a key component of fungal cell membranes. Antiprotozoan drugs may work by increasing reactive oxygen species or interfering with DNA replication, while antihelminthic drugs can disrupt microtubules or block neuronal transmissions. The problem is exacerbated in antiviral drugs because viruses replicate within human host cells, making selective targeting even more difficult. Moreover, the rapid evolution of resistance among pathogens, such as in the case of anti-malarial drugs against Plasmodium species, further complicates chemotherapy.
For instance, while treating protozoan infections of the skin and eyes, drugs must be specific to the pathogen, because of the vast diversity among protozoa. And when treating helminthic infections, it is necessary to use drugs that eliminate adult worms without severely affecting the host. Overall, the challenges of chemotherapy in treating viral, fungal, protozoan, and helminthic infections stem from the need for selective toxicity, which is difficult to achieve due to similarities between pathogen and host cells, or because of the intracellular nature of some pathogens.