The general rule is that a landlord is not liable for injuries caused by a tenant's dog, unless the landlord knew the dog was dangerous. (Uccello v. Laudenslayer (1975) 44 Cal.App.3d 504, 510-511, 118 reversed nonsuit in favor of landlord, where it could be assumed from inferences from the opening statement that the landlord knew the dog was vicious, and landlord could have abated the problem by terminating the tenancy.) Exceptions to this rule have developed, e.g., where the injury occurs on a part of the premises over which the landlord retains control (such as common hallways, etc.). (Uccello, supra, 44 Cal.App.3d at p. 511.) "A common element in these exceptions is that either at or after the time possession is given to the tenant the landlord retains or acquires a recognizable degree of control over the dangerous condition with a concomitant right and power to obviate the condition and prevent the injury. In these situations, the law imposes on the landlord a duty to use ordinary care to eliminate the condition with resulting liability for injuries caused by his failure so to act. " (Ibid.) "Public policy requires that a landlord who has knowledge of a dangerous animal should be held to owe a duty of care only when he has the right to prevent the presence of the animal on the premises. Simply put, a landlord should not be held liable for injuries from conditions over which he has no control. On the other hand, if a landlord has such a degree of control over the premises that it fairly may be concluded that he can obviate the presence of the dangerous animal and he has knowledge thereof, an enlightened public policy requires the imposition of a duty of ordinary care. To permit a landlord in such a situation to sit idly by in the face of the known danger to others must be deemed to be socially and legally unacceptable." (Id. at pp. 511-512.) However, Uccello, supra, 44 Cal.App.3d 504, also said, "It should be emphasized that a duty of care may not be imposed on a landlord without proof that he knew of the dog and its dangerous propensities. Because the harboring of pets is such an important part of our way of life and because the exclusive possession of rented premises normally is vested in the tenant, we believe that actual knowledge and not mere constructive knowledge is required." (Id. at p. 514.) In a footnote, Uccello added a landlord's actual knowledge "may be shown, not only by direct evidence, but also by circumstantial evidence. Hence his denial of such knowledge will not, per se, prevent liability. However, actual knowledge can be inferred from the circumstances only if, in the light of the evidence, such inference is not based on speculation or conjecture. Only where the circumstances are such that the defendant 'must have known' and not 'should have known' will an inference of actual knowledge be permitted. " (Id. at p. 514, fn. 4.) In Lundy v. California Realty (1985) 170 Cal.App.3d 813, the plaintiff entered the tenant's backyard (there was no fence preventing access from the front of the house to the backyard), and injured himself jumping over a fence when the dog came running toward him. (Id. at p. 817.) Aside from knowing the tenants kept on the premises a shepherd dog named Thunder (as stated in the rental agreement), the landlords knew nothing about the dog, and therefore the facts before the trial court fell short of creating a triable issue of fact as to the landlords' knowledge of any dangerous propensities on the part of the dog. (Id. at pp. 818-822.) A landlord's liability for injuries caused by a tenant's dog following escape from the landlord's premises is determined by whether the landlord knew of the dog's vicious propensity and had the ability to prevent the attack. (Donchin v. Guerrero (1995) 34 Cal.App.4th 1832.) Guard dogs on commercial property are treated differently. In Portillo v. Aiassa (1994) 27 Cal.App.4th 1128 (Portillo), the Sixth Appellate District affirmed a jury verdict against a landlord, in favor of a plaintiff who, while making a delivery to a liquor store, was attacked by a guard dog owned by the person who operated the store. Portillo distinguished Uccello, supra, 44 Cal.App.3d 504, and Lundy, supra, 170 Cal.App.3d 813, stating, "Those cases involved a family pet kept in a single-family residence. In contrast here the animal was a guard dog in a liquor store which was open to the public. The risk that someone would be seriously injured was far greater in the instant case. Moreover, while we recognize that particular neighborhoods or circumstances may require the presence of guard dogs for safety reasons, there is no strong public policy in favor of guard dogs in public places. Such animals cannot be classified as 'an important part of our way of life' in the same way that pets can. " (Portillo, supra, 27 Cal.App.4th at pp. 1137-1138.) Portillo said the landlord was aware the commercial tenant was keeping a dog in a business open to the public and, in the exercise of reasonable care, the landlord would have seen on the premises a sign warning of the guard dog and a newspaper article posted near the door, describing the guard dog's recent attack on an attempted robber in the store. (Id. at pp. 1132, 1138.)