# Important Roles of Oligo- and Polysaccharides
## 1 Introduction  
Various types of antiviral drugs are being examined against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), but unfortunately, several of these candidates are often toxic and have several side effects [1, 2]. Therefore, it is imperative to search for virvicidal drugs with low or no toxicity that irreversibly inhibit viruses. In this regard, various naturally occurring compounds have been evaluated for their potential inhibitory activities against viral infections [2, 3]. Among them, natural polysaccharides are effective inhibitors of assorted pathogenic viruses, such as influenza A virus, human cytomegalovirusovirus (HCMV), herpes simplex virus (HSV), human immunodeficiency virus (HIV), dengue virus (DENV), respiratory syncytial virus (RSV), human metapneumovirus (hMPV), severe acute respiratory syndrome coronavirus (SARS-CoV), and Middle East respiratory syndrome-related coronavirus (MERS-CoV) [4, 5, 6]. For instance, sulfated polysaccharides were examined as topical microbicidal agents in medical trials to thwart viruses-triggered sexually transmitted diseases (STDs). Accordingly, various polysulfate compounds have shown promising potentials for inhibition of the virus replication by obstructing the virion attachment to the host cell [4, 7]. However, the efficacy of these compounds in vivo against these viral infections has some important challenges and drawbacks, including poor absorption, failure to reach target tissues, noxious side effects, and anticoagulant characteristics. Moreover, there is a demand for probing new materials, especially for the drug resistance to antiviral agents during therapy [4, 7]. Importantly, the intricacy of marine polysaccharides and their derivatives and their structural variety are responsible for their antiviral actions in diverse stages of various viral infection procedures (Figure 1) [8]. Different types of seaweed polysaccharides, comprising agarans, ulvans, fucoidans, carrageenanans, laminarins, and alignates, have exhibitedappropriate antiviral effects, and their promising potentials for therapeutic appliances are garnering remarkable attention [8]. These materials with distinctive structures exert antiviral effects via interference at various phases of the viral infection cycle. Additionally, polysaccharides and, especially, sulfated polysaccharides are endowed with significant polyanionic attributes and can, thus, be employed for blocking the cationic charge on the surface of cells to prevent the virus assault or even adsorption [9]. These sulfated polysaccharides have significant therapeutic potentials, as they can imitate sugar-rich molecules, glycosaminoglycans, prevailing in the membranes of the cells. These glycosaminoglycans such as dermatan sulfate, chondro sulfate, and heparan sulfate are vital in mammalian compositions. As an example, receptors of heparan sulfate on the surfaces of the cells are critical in various pathological and physiological processes, besides being necessary for the entry of the virus into vulnerable cells. It has been noted that sulfated polysaccharides compete for binding sites usually used by glycosaminoglycans, thus inhibiting these actions [10; 11]. The preventive action of natural sulfated polysaccharides varies with the compound and the virus. In one study, three different forms of carrageenan G3d and the dl-galactan hybrid C2S-3 (two homogeneous sulfated polysaccharides), accessible from the red seaweeds _Gymnogrus stiffithsiue_ and _Cryptonemia crenulata_, were evaluated for their virucidal potentials against four serotypes of DENV in various host cell types [12]: both seaweed derivatives had specific inhibitory effects against DENV-2 multiplication in Vero cells (selectivity indices >1000 and half maximal inhibitory concentration (IC\({}_{50}\) = 1 ug mL\({}^{-1}\))).