Polysaccharides, especially sulfated polysaccharides, can directly interfere with viral replication related enzymes and relevant targets in host cells. Iota-carrageenan can effectively inhibit porcine reproductive and respiratory syndrome virus (PRRSV) replication at mRNA and protein levels in both Marc-145 cells and porcine alveolar macrophages [98]. Carrageenan oligosaccharide and its sulphated derivative have good inhibitory effects on IAV replication both in vitro and in vivo, while not seem to be dependent on the interferon system [99]. Sulfated polysaccharide from Gracilaria lemaneiformis shows anti-influenza virus activities in vitro by inhibiting viral adsorption and replication on host cells [100]. Polysaccharides isolated from Grifola frondosa showed resistance against enterovirus 71, a positive-stranded RNA virus, by blocking viral replication and inhibiting viral VP1 protein expression and genomic RNA synthesis [101]. The virus replication was inhibited by a sulfated polysaccharide from Angelica sinensis, which is a commonly used traditional Chinese herbal medicine, at the dose of 10 and 30 mg/kg (26% and 30% inhibition respectively) [102]. APS has a long-lasting inhibitory effect on HBV replication in vivo, which can be used as a supplementary modality to treat hepatitis B infection [103]. Furthermore, APS can inhibit the replication of avian IBV in vitro in a dose-dependent manner [78].